Codeforces Round #849 (Div. 4) Editorial

#	User	Rating
1	tourist	3775
2	Benq	3724
3	orzdevinwang	3697
4	Radewoosh	3651
5	jiangly	3632
6	cnnfls_csy	3620
7	-0.5	3545
8	inaFSTream	3478
9	fantasy	3468
10	Rebelz	3415

#	User	Contrib.
1	adamant	178
2	awoo	167
3	BledDest	165
4	Um_nik	163
5	maroonrk	162
6	SecondThread	160
7	nor	158
8	-is-this-fft-	154
9	kostka	146
10	TheScrasse	144

We hope you enjoyed the contest!

Tutorial

Solution

#include <bits/stdc++.h>

using namespace std;

const string s = "codeforces";

void solve() {
	char c;
	cin >> c;
	for (char i : s) {
		if (i == c) {cout << "YES\n"; return;}
	}
	cout << "NO\n";
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int tt; cin >> tt; for (int i = 1; i <= tt; i++) {solve();}
	// solve();
}

1791B - Following Directions

Idea: flamestorm

Tutorial

Solution

#include <bits/stdc++.h>

using namespace std;

const int MAX = 200007;
const int MOD = 1000000007;

void solve() {
	int n;
	cin >> n;
	string s;
	cin >> s;
	int x = 0, y = 0;
	for (int i = 0; i < n; i++) {
		if (s[i] == 'L') {x--;}
		if (s[i] == 'R') {x++;}
		if (s[i] == 'D') {y--;}
		if (s[i] == 'U') {y++;}
		if (x == 1 && y == 1) {cout << "YES\n"; return;}
	}	
	cout << "NO\n";
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int tt; cin >> tt; for (int i = 1; i <= tt; i++) {solve();}
	// solve();
}

1791C - Prepend and Append

Idea: flamestorm

Tutorial

Solution

#include <bits/stdc++.h>

using namespace std;

const int MAX = 200007;
const int MOD = 1000000007;

void solve() {
	int n;
	cin >> n;
	string s;
	cin >> s;
	int l = 0, r = n - 1, ans = n;
	while (s[l] != s[r] && ans > 0) {l++; r--; ans -= 2;}
	cout << ans << '\n';
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int tt; cin >> tt; for (int i = 1; i <= tt; i++) {solve();}
	// solve();
}

1791D - Distinct Split

Idea: SlavicG

Tutorial

Solution

#include "bits/stdc++.h"
using namespace std;
 
#define ll long long

#define          all(v)              v.begin(), v.end()
#define         rall(v)              v.rbegin(),v.rend()
 
#define            pb                push_back
#define          sz(a)               (int)a.size()

void solve() {
    int n; string s; cin >> n >> s;
    vector<int> cnt(26, 0), p(26, 0);
    for(auto x: s) cnt[x - 'a']++;
    int ans = 0;
    for(auto x: s) {
        --cnt[x - 'a'];
        ++p[x - 'a'];
        int cur = 0;
        for(int i = 0; i < 26; ++i) {
            cur += min(1, cnt[i]) + min(1, p[i]);
        }
        ans = max(ans, cur);
    }
    cout << ans << "\n";
}   
 
int32_t main() {
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    int t = 1;
    cin >> t;
    while(t--) {
        solve();
    }
}

1791E - Negatives and Positives

Idea: SlavicG

Tutorial

Solution

#include <bits/stdc++.h>
using namespace std;

int main() {
    int t; cin >> t;
    while(t--) {
        int n; cin >> n;
        vector<int> a(n);
        long long sum = 0;
        int negs = 0;
        for(int i = 0; i < n; ++i) {
            cin >> a[i];
            if(a[i] < 0) {
                ++negs;
                a[i] = -a[i];
            }
            sum += a[i];
        }
        sort(a.begin(), a.end());
        if(negs & 1) sum -= 2 * a[0];
        cout << sum << "\n";
    }
}

1791F - Range Update Point Query

Idea: flamestorm

Tutorial

Solution

#include <bits/stdc++.h>
using namespace std;

int digit_sum(int n) {
    int ret = 0;
    while(n) {
        ret += n % 10;
        n /= 10;
    }
    return ret;
}
void solve() {
    int n, q; cin >> n >> q;
    vector<int> a(n);
    set<int> s;
    for(int i = 0; i < n; ++i) {
        cin >> a[i];
        if(a[i] > 9) s.insert(i);
    }
    while(q--) {
        int type; cin >> type;
        if(type == 1) {
            int l, r; cin >> l >> r; --l, --r;
            int lst = l;
            while(!s.empty()) {
                auto it = s.lower_bound(lst);
                if(it == s.end() || *it > r) break;
                a[*it] = digit_sum(a[*it]);
                int paiu = *it;
                s.erase(it);
                if(a[paiu] > 9) s.insert(paiu);
                lst = paiu + 1;
            }
        } else {
            int x; cin >> x; --x;
            cout << a[x] << "\n";
        }
    }
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(0);
	int t; cin >> t;
	while(t--) {
	    solve();
	}
}

1791G1 - Teleporters (Easy Version)

Idea: flamestorm

Tutorial

Solution

#include "bits/stdc++.h"
using namespace std;
 
#define ll long long

#define          all(v)              v.begin(), v.end()
#define         rall(v)              v.rbegin(),v.rend()
 
#define            pb                push_back
#define          sz(a)               (int)a.size()

void solve() {
    int n, c, ans = 0; cin >> n >> c;
    priority_queue<int> q;
    for(int i = 1, x; i <= n; ++i) {
        cin >> x;
        q.push(-x - i);
    }
    while(!q.empty()) {
        int x = -q.top(); q.pop();
        if(x > c) break;
        ++ans;
        c -= x;
    }
    cout << ans << "\n";
}   
 
int32_t main() {
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    int t = 1;
    cin >> t;
    while(t--) {
        solve();
    }
}

1791G2 - Teleporters (Hard Version)

Idea: flamestorm

Tutorial

Solution

#include <bits/stdc++.h>
#define startt ios_base::sync_with_stdio(false);cin.tie(0);
typedef long long  ll;
using namespace std;
#define vint vector<int>
#define all(v) v.begin(), v.end()
#define MOD 1000000007
#define MOD2 998244353
#define MX 1000000000
#define MXL 1000000000000000000
#define PI (ld)2*acos(0.0)
#define pb push_back
#define sc second
#define fr first
#define int long long
#define endl '\n'
#define ld long double
#define NO cout << "NO" << endl
#define YES cout << "YES" << endl
int ceildiv(int one, int two) {if (one % two == 0) {return one / two;}else {return one / two + 1;}} int power(int n, int pow, int m) {if (pow == 0) return 1;if (pow % 2 == 0) {ll x = power(n, pow / 2, m);return (x * x) % m;}else return (power(n, pow - 1, m) * n) % m;} int gcd(int a, int b) { if (!b)return a; return gcd(b, a % b);} int factorial(int n, int mod) {if (n > 1)return (n * factorial(n - 1, mod)) % mod; else return 1;} int lcm(int a, int b) {return (a * b) / gcd(a, b);} vector<int> read(int n) {vector<int> a; for (int i = 0; i < n; i++) { int x; cin >> x; a.pb(x);} return a;}struct prefix_sum{vint pref;void build(vint a){pref.pb(0);for(int i = 0; i < a.size(); i++){pref.pb(pref.back()+a[i]);}}int get(int l, int r){return pref[r]-pref[l-1];}};//mesanu

void solve()
{
    int n, c;
    cin >> n >> c;
    vector<pair<int, int>> a;
    for(int i = 0; i < n; i++)
    {
        int x;
        cin >> x;
        a.pb({x+min(i+1, n-i), x+i+1});
    }
    sort(all(a));
    vector<int> pref;
    pref.pb(0);
    for(int i = 0; i < n; i++)
    {
        pref.pb(pref.back()+a[i].fr);
    }
    int ans = 0;
    for(int i = 0; i < n; i++)
    {
        int new_c = c-a[i].sc;
        int l = 0, r = n;
        int mx = 0;
        while(l <= r)
        {
            int mid = l+r>>1;
            // Calculate price
            int price = pref[mid];
            int now = mid+1;
            if(mid > i)
            {
                price-=a[i].fr;
                now--;
            }
            if(price <= new_c)
            {
                mx = max(now, mx);
                l = mid+1;
            }
            else
            {
                r = mid-1;
            }
        }
        ans = max(ans, mx);
    }
    cout << ans << endl;
}

int32_t main(){
    startt
    int t = 1;
    cin >> t;
    while (t--) {
        solve();
    }
}

include

using namespace std;

int main(){ long i; cin>>i; while(i--){ long a; cin>>a; long first = 0; long mid = a; string te; cin>>te; unordered_map<char,long> temp; for(long i = 0; i<a; i++){ if(temp[te[i]]==0){ temp[te[i]]++; first++; } else{ mid = i; break; } } long second = 0; unordered_map<char,long> temp1; for(long i = mid; i<a; i++){ if(temp1[te[i]]==0){ temp1[te[i]]++; second++; } } cout<<first+second<<endl; } return 0; }

#include<bits/stdc++.h> using namespace std; void solve() { int n; string s; cin>>n>>s; map<char,int>p; int x = 0; for(int i = 0;i < n;i ++ ) { p[s[i]]++; if(p[s[i]]==2) { x = i; break; } } string ss; for(int i = x;i < s.size();i++) ss+=s[i]; int sum = 0; sort(ss.begin(),ss.end()); for(int i = 0;i < ss.size();i ++ ) { if(ss[i]!=ss[i+1]) sum++; } cout<<x+sum<<endl; } int main() { int t; cin>>t; while(t--) solve(); return 0; }

#include <bits/stdc++.h> #define MAXN 200005 using namespace std; int n,q,arr[MAXN],diff[MAXN],opera; int lowbit(int x) { return x&(-x); } void add(int x,int v) { for (int i=x;i<=n;i+=lowbit(i)) diff[i]+=v; return; } int ask(int x) { int ans=0; for (int i=x;i>=1;i-=lowbit(i)) ans+=diff[i]; return ans; } int main () { int T; cin>>T; while (T--) { cin>>n>>q; for (int i=1;i<=n;i++) { cin>>arr[i]; diff[i]=0; } for (int i=1;i<=q;i++) { cin>>opera; if (opera==1) { int x,y; cin>>x>>y; add(x,1); add(y+1,-1); } else if (opera==2) { int x,y; cin>>x; y=min(ask(x),3); int ans=arr[x],temp; for (int j=1;j<=y;j++) { temp=0; while (ans) { temp+=ans%10; ans/=10; } ans=temp; } cout<<ans<<'\n'; } } } return 0; }

I am a beginner... problem E, help... find the largest sum... if we have: 6 1 1 -1 2 -2 -2 -1 -1 -1 2 -2 -2 -1 1 1 2 2 2 result is 8... I'm wrong, it kills me the doubt help.... because if we have 5 0 5 -5 1 4 0 -5 -5 1 4 0 5 5 1 4 result is 15 help please what am I doing wrong...

include<bits/stdc++.h>

using namespace std; int main(){ int t; cin>>t; while(t--){ int n,res=0,sum=0; cin>>n; int count=0; string s; cin>>s; sets1; sets2; sets3; for(int i = 0; i<n; i++){ s3.insert(s[i]); } if(s3.size()==1){ cout<<"2"<<endl; } else{

for(int i =0; i<n-1; i++){ if( s[i]==s[i+1]){ count = i+1; } if(i==n-2){ sum=i; break; } } if(sum==n-2){ for(int j = 0 ; j<n-1; j++){ if((s[res]==s[j+1]) && (res!= (j+1))) { count = j+1; } else{ if(j==n-2){ res++; j=0; } } } } for(int k = 0; k<count; k++){ s1.insert(s[k]); } for(int g = count; g<n; g++){ s2.insert(s[g]); } cout<<s1.size()+s2.size()<<endl; } } return 0;

} ~~~~~

Comments (225)

Write comment?

brownfox2k6

8 months ago, # |

-6

wow tutorial comes out so fast!

→ Reply

speedy_boy

8 months ago, # ^ |

+21

wow comment comes out so fast!

JOIN_GROUP

wow reply comes out so fast!

Sanat04

← Rev. 2 →

-10

safin17.8

wow i am not fast

rockkyy

wow my reply comes so fast

GOD_OF_HELL

wow i am not so fast than you

Shyam2811

wow my net works so fast during the contest

XiaoZhuge

6666

LittleXi

wow you are so fast

spectacle

-22

话说666是不是只有中国有

northchen

确实

masplove

-17

Why many people vote "I don't like it"?Is it because I used Chinese?

rajneeshanshu

i'm so slow

可能因为他们看不懂，hhh

pratikbhagat2707

Even i am also fast

笑死了，发个中文被踩了10次（

My contribution drops so fast!

我从+1掉到-1了。。。

问题不大，分不掉就行（

aclaputra

I'm slow

__LAKSHAY__

7 months ago, # ^ |

I am average

B_Consistent

4 months ago, # ^ |

wow upvote comes so fast

aydv_

Wow reply to a reply comes out so faist!

AnkitMr

wow the series grows so fast

KareemBenzeema

wow you can type so fast

ayush_tiwari26

5 months ago, # ^ |

wow 3 months went by so fast!

Losa1

wow my rating drops so fast!

hwaiting_dk

wow what a thread

mersenne-twister

Kudos for the fun round and the near instantaneous posting of the tutorial.

tenick

can someone help me understand why my code for problem G1 doesn't work? https://mirror.codeforces.com/contest/1791/submission/192028236

EDIT: nvm i figured it out, using a set will remove duplicates siiiiggghhhhhhhhhhhhhhhhhh

u_s_m_a_n

I think you should read the question again and Test case explanation also.

matson5740

Did the same thing on my first submission. Multiset or a sorted vector would’ve worked

18o3

For problem F we can also use a segment tree where the nodes have the max value in the subtree. If the maximum value in any subtree is less than 10 then we shouldn't update that subtree and can skip it. Complexity wise it should be the same as the editorial solution. Implementation 192043154

morodmiya21

Bro can you please tell me what is wrong in this implementation of seg tree? Code

variety-jones

Take a look at Ticket 16708 from CF Stress for a counter example.

Bro Thank you very much. I found my mistake. Also you shared a nice website. Thanks a lot

Activate my subscription Please

It's active now. Please note that it takes upto 24 hours for activation.

wlh666

2 months ago, # ^ |

I think it would be better to solve the F problem by using the union lookup set optimization https://mirror.codeforces.com/contest/1791/submission/214420789

jkulanko

Perfect div 4 round IMO. Every problem was solvable by div 4 people, with 1 problem that would push most div 4 people and another problem that would really push most div 4 people but still was solvable

LZYu

Ah man, I'm kinda disappointed with myself tho I only managed 3 questions :/

← Rev. 4 →

Did anyone else solve f with DSU? My solution: https://mirror.codeforces.com/contest/1791/submission/192000576

Basically if i and i-1 are < 10, we can put them the same set. If a query has an l and r in the same set, we don't need to do anything

PrateekTewary

Yes. I did the same. F with DSU.

Rajveer_100

I solved problem F with segment tree (lazy propagation), where I keep track of each update range [l, r] to know the number of times a certain index i [l <= i <= r] also ensuring when the number x <= 9 to break the loop (:)committed 2 WAs initially because of this).

Code

Bro can you please tell me what is wrong in this implementation of seg tree? 192042203

mykhail0

unfortunately i solved it using lazy propagation segtree too. I had a feeling it's not needed since it's a div 4 round, and the solution without it is nice. Segtrees sure are powerful though

rav_ish

It can be solved using square root decomposition which i used during the contest

why_I

A great problemset.

btw

nick.sh

Congrats!

Thank you 😄

thermite104

I want this kind of flex

SeanInAction

Wow, the tutorial came out so fast. Searched for useful data structures online for problem F and found that using binary indexed tree works.

nice,thanks to you i found out lazy propagation is possible with BIT

vantaablackk

Short and succinct solutions. Love it 👏

tn757

Very cool that F can be solved with set and DSU. I got stuck on BIT implementation because I don't have a template for it and have only solved a few CSES problems using it. Forgot about the limitation of 3 change operations. Thought G2 would be DP, but the binary search solution is quite elegant. 10/10 problemset overall.

29logN

Solved E with DP. Also for the first time I have used segment tree lazy propagation in a live contest (problem F) :)

PLAn22

Can you show me your code for E? i spend an hour to find that a O(n) solution.And i just know if i use dp to solve E i will be TLE.

sure

    ll n; cin>>n;
    
    ll ara[n+1];
    
    for(ll i=1; i<=n; i++)
    {
    	cin>>ara[i];
    }
    
    ll dp[n+1][2];
    
    memo(dp,0);
    
    dp[1][0] = ara[1];
    dp[1][1] = -ara[1];
    
    for(ll i=2; i<=n-1; i++)
    {
    	dp[i][0] = max(dp[i-1][0] + ara[i], dp[i-1][1] - ara[i]);
    	dp[i][1] = max(dp[i-1][0] - ara[i], dp[i-1][1] + ara[i]);
    }
    
    dp[n][0] = max(dp[n-1][0] + ara[n], dp[n-1][1] - ara[n]);
    
    cout<<dp[n][0]<<endl;

coderdhanraj

Nice! I was trying with dp too but couldn't make it AC with dp. 191959517

Doooog

← Rev. 3 →

Wow, I also use segment trees for F

Bebo

Everywhere i go i see "In queue"

I wonder what role mesanu and Mike had in preparation of this round given that all problem ideas were by Slavic and flamestorm. Maybe they helped prepare the test data or something.

daifuku

Thanks to y'all for amazing contest!

For those who solved F with segment tree/fenwick tree, etc., Um_nik's 2nd and 3rd bullet point from this comment is highly relevant 😁

Everyone is not Um_nik. May be they tried to think of not using Fen-wick but did not get idea of binary search As a last resort they might have used fen-wick

yeah true, but gotta do what you gotta do if it's the only solution you came up with

regain0002

Can anyone hack my G2? I think it is not correct even I passed the pretest. 192031288

Okay~ I hacked myself and other four Unfortunate guys

Legend

somaigaurav

orz

prakalpmanav1711

DISTINCT SPLIT plz tell why my code doesn't work. If possible, plz mention the test case where it fails as well.

Aryan0Barsaiyan

can someone help me understand why my code for problem F gives tle? https://mirror.codeforces.com/contest/1791/submission/191999834

bjy

The else case of a[x] < 9 allows for repeatedly applying fun(9).

thanks bro got it

-29

Pirated_Codes

enough?

franklidev

This is my first contest on this platform and it's really great.

For G2, Editorial solves in O(nlogn) after sort but I solve in O(n) + O(logn) after sort.

https://mirror.codeforces.com/contest/1791/submission/192046876

Dominater069

thats the same complexity sir

balakrishnan

My screencast with explanations. The problems were very interesting. Thanks for the contest.

XHYMathematics

It seems that the problem writers like binary indexed tree. Both F and G2 can be solved by binary indexed tree.

123_why_123

did someone do the g2 question using priority queue?

I tried, but it ended up highlighting how muddy my idea of picking the first from-left move was... rather than sorting that out (perhaps by literally sorting), I got lost in the weeds of popping and replacing per case/situation and then deleting/rewriting messes.

kcbiradar

good round!

W_Franklin

I think the F problem can also be solved with segment tree. https://mirror.codeforces.com/contest/1791/submission/192019842

Hasan__Zia

Bro, What did you do?? Can you explain me please...

shalekberli

Btw there is also fenwick/segment tree solution for problem F. That is even really simple to implement it.

TSTazbid

Nice contest, and thanks for fast editorial.

E404_Not_Found

There's a sqrt decomposition solution for F, for a range of size sqrt(n) save the number of updates on this rsnge‌ as cnt of that range, and then for the queries which they want you to print the value of an index, add cnt of that index + cnt of the range which it belongs to (let sum of these be $$$K$$$) and find the $$$K$$$'s digit sum for that index's number, here's the code :. 192059676

34z12000

Can someone explain this part in problem F to me on example: 1 l r — search for the smallest active index at least l (since the list is sorted, we can do it in O(logn)). Afterwards, update that index (replace ai with S(ai)), remove it if it's no longer active, and binary search for the next largest active index in the sorted list, until we pass r.

Is it possible to solve the problem F using Segment tree algorithm concept?? Can anyone help me with this solution using Segment tree...

not_just_rr

Solved it using segment trees but got TLE in tc 4

hydra_cody

Precompute the sum of digits till there remains one digit. It helps me in fighting with TLE on 4.192204290. I use bit.

i did it but still got TLE

nk_lk

The edu section has a guide to such segment tree

Zadymiarz1234

Sure, it can. You need simple tree with 'modification on interval' and 'single element access' operations. Check out this blog for implementation details https://mirror.codeforces.com/blog/entry/18051

HNOONa

For problem E, "if the count of negative numbers is odd, we must have one negative number at the end How can i prove this?

here u can shift negative sign to any elements like if i have -2 3 -1 4 -6 5 then it can be easily changed to 2 -3 -1 4 -6 5 then 2 3 1 4 -6 5 still u have one negative now shift that negative sign to 3 index (1 based)

I understand that negative signs can be shifted anywhere.

What im asking is how do we know that there is no sequence of operations such that no negative numbers remain (in the odd case)?

in one operations either u remove two negative sign or no negative signs see there are three cases when both are negative :- make both positive, 2 neg signs removed when both are postive : no needed , 0 neg signs removed when one is neg and one is positve : still no neg sign removed so every time numbers of neg sign removed is multiple of 2 hence it count is odd one sign always remains

Why my code fail at testCase 3 in F.

Take a look at Ticket 16719 from CF Stress for a counter example.

what is the proof of greedy for G1. I thought of it like knapsack but constraints were big so i did greedy but I dont know why greedy is working. shouldn't it fail for the same reason where greedy doesnt work for knapsack ?

IanISam

Knapsack has a value and weight. If there is only a value your can use greedy.

Thought that each number could only have 3 states for F. This is so sad

Guys, pls, explain F

I used sweep line on F, I haven’t seen anyone talk about it yet. Store the queries and the index they appear and start the sweep line on the array from left to right. Add active elements(index of the query) when it reaches l in an order set and remove them after it reaches r+1. The first 3 values indicate the minimum index to achieve that state. Can be optimized to o(n)

anuj_negi

Used the same Alg and got HACKED. Possibly TLE.

You have to process all the queries beforehand

I actually realized what you did here (Checked your solution). At first, it was uneasy to comprehend because of Java, but you did a mixed sweep line with the ordered set.

That was a great one and to check them 0,1,2,3 for later is great.

But if consider a problem with 10 states, it will consume memory. [Though Imma PY coder XD]

Hack

Was able to hack my solution to about over half the memory and time available with Java. So it does consume a lot of memory but it is possible to reduce the amount of memory and time used by not storing the states the numbers can transition. So about n+4q memory.

Great one at hack and further possibilities. You got this. :))

Thanks :)

Nvm about o(n) still o(nlog(q)+q)

HaydarFakhoury

192071106 I'm stuck wondering why this code is not working. Not sure if I misunderstood the problem.

Nishant_Mohanty

Iterate forward and backward separately. you will see a pattern.

s010456

Try this testcase:

1
8
bcaffggf

Expected output:7

Sylvester_

Got my precious 2 solution Hacked due to Arrays.sort(JAVA) so much pain this days :-(

Yeah, that and unordered sets are really annoying hacks.

idkwmirn

In problem G2, why is it wrong to take the first portal as portal having minimum overall cost? Can you give a failing test case?-

judgme_nt

If I understood your question, it's because that minimum could be taken with an even lower cost.

For example for $$$n=3$$$, $$$a = [17, 8, 2]$$$ and $$$c = 13$$$ we can get use second and third teleporter but not if I go to the third one first.

Thanks! Got it

timafefi

On problem D I stumbled upon a very weird thing:

When I test my code on my computer, all the test input returns correct answers. However, when I submit this code, I get an error on string "paiumoment". I have no idea how to debug this, because on my machine the answers are all correct. Does anybody know what is the issue?

My g++ version is 12.2.0. I stried both versions 17... and 14.6.4.0 when I submitted the code.

Here's the link to my code: https://mirror.codeforces.com/contest/1791/submission/192076794

P.S. I have a feeling that it does something to do with g++ versions, because on different versions the incorrect answers are different values. But how exactly does it work? I never came across such a problem before

bronze_coder

You never initialized cnt which causes undefined behavior because the values of cnt do not start at 0.

Oops, that's an awkward mistake... Thanks a lot!

yadavDipesh

would not have happened in java

Weak_Password

Wow tutorial comes out so fast!

Can problem F be solved with a segment tree?

dani450

I can't figure out what is wrong with my code for problem 1791D — Distinct Split. Can anyone help me out? #192086640

fernandezpalomojosue

I solved the F with a segment tree over a difference array

RasecBadguy

Can you explain your approach?

wangkm123

I don't know where is wrong about my D code ?

I need help !!!

atharvd

I know some people used a seg tree over a difference array, but did anyone else have the same idea but do it with a fenwick tree? https://mirror.codeforces.com/contest/1791/submission/192095662

akshat___17

Can anyone please tell me why this code is giving wrong ans on testcase 2 For the F problem https://mirror.codeforces.com/contest/1791/submission/191982272

Take a look at Ticket 16718 from CF Stress for a counter example.

cercatrova

In G1, what I understand, there is no port in point 0. So, according to the problem, I cannot teleport back to 0 from 0. But if I consider there is no port at point 0, I get WA. To get AC, I had to consider that there could be port at 0. Why? Also the explanation for testcase 1 is given considering the array is 0 indexed. But while explaining testcase 2, we are considering the array is 1 indexed. I am having confusion. Please help me!

pcajourney

Array is 1 indexed every where , maybe you're misreading the sample tests. And also ,there is no port at 0

Oh I thought 0 index is 0 point. My bad

krishna110

// cur += min(1, cnt[i]) + min(1, p[i]);

In the editorial why are we taking the minimum of frequency of a character in a and b ?

I am sorry I didn't mention this doubt is from problem D

bunkerman

we want distinct letters i.e., the frequency should only be 0 or 1.

ashutosh_saini

Can somebody explain why set solution is working in F while map solution giving TLE;

Here is my map Solution giving TLE; https://mirror.codeforces.com/contest/1791/submission/191980859

rsan

Problem E solution using DP in O(n). Refer here

a_mentalist

Please help me I am not able to recognize my mistake Please help me think i fault in my logic for problem D I am stopping my iteration in string at the point where I find a duplicate character for example : if string is ppzs the two string according to my algorithms should be p + pzs

Take a look at Ticket 16714 from CF Stress for a counter example.

@variety-jones how do u use CF stress do u have a paid subscription?? thank u for ur help

I'm the creator of the website.

Yes, if you'd like to use it for yourself, you will need to purchase a subscription.

yeah the website is really very cool

Hazer_BJU

Problem F can also be solved by binary indexed tree, which is much easier to implement than segment tree if you didn't come up with the tutorial solution.

Rahul_abr

Can anyone explain to me how to solve G2? Thanks in advance.

Aarav

Problem F can be solved with simple seg tree. On each nodes of seg tree we store the count of times that range has been updated by first type of query. Then for the second type of query, we keep adding the nodes value until we reach the desire x. If the total no. of count is >=3 then we simply output our precalculated value for index x till 3,(you can see this precalculation in my solution, its too easy to explain). Other wise output precalculated[count][x].
You can refer to my solution for more clarification 192045935

TrendBattles

Great contest! I love it! Hope I can get specialist this time.

Krutik-patel

Can someone help me understand why my submission https://mirror.codeforces.com/contest/1791/submission/192109277 for G2 is giving WA on test 3

Take a look at Ticket 16713 from CF Stress for a counter example.

Once_I_Liked_AGirl

I have a strong data for problem F, but I forget to try it when open hacking,but it can successfully hack one of my friends's code,I left it here but I'm so sorry for that it's a chinese website,but I have tried my best to explain how to use it,you can just download data at the bottom of the website. here: https://www.luogu.com.cn/problem/U279041

I'm so sorry for don't konw how to upload file on codeforces.

shivam_360

can anyone explain why i am getting tle one test case 3 on problem F-

Spoiler

ll testcases=1; cin>>testcases;

while(testcases--)
{
    ll n,q;
    cin>>n>>q;
    vector<ll>arr(n,0);
    set<ll>st;
    for(int i=0;i<n;i++)
    {
        cin>>arr[i];
        if(arr[i]>=10)
        {
            st.insert(i);
        }
    }
    while(q--)
    {
        ll type;
        cin>>type;
        if(type==1)
        {
            ll l,r;
            cin>>l>>r;
            l--;
            r--;
            while(true)
            {
                if(st.empty())
                {
                    break;
                }
                auto it=st.lower_bound(l);
                if(it==st.end())
                {
                    break;
                }
                if((*it)<=r)
                {
                    ll temp=arr[(*it)];
                    ll curr=0;
                    while(temp)
                    {
                        curr+=(temp%10);
                        temp/=10;
                    }
                    arr[(*it)]=curr;
                    if(curr<10)
                    {
                        st.erase(it);
                    }
                    l=(*it)+1;
                }
                else{
                    break;
                }
            }
        }
        else{
            ll ind;
            cin>>ind;
            ind--;
            cout<<arr[ind]<<endl;
        }
    }
}

Venturo

E using DP "Memoization" Code

UtTaRaN

Can somebody please explain the binary search section of G2?

I did similar approach up to it, then used modified inbuilt upper_bound function. It was my logical error. But I still cant understand from the editorial.

Men3m

Can someone tell me why this code get TLE in F mySubmission

3.14159265359

You using SegmentTree, so when you update range [l, r], you update from [l, r]. If we have 1E5 query (1 1 n), your code has time complexity is O(n * 1E5 * log(n)) => TLE. This is my opinion about your code.

I think as i know, in the segment tree the one query takes log(n), and hence the queries over all test cases won't exceed 2e5 so the total complexity will be the number of queries multiplied by the time per query plus the construction of the tree (build function) so we can say: O(2e5 * log(2e5) + 2e5) = O(C * 1e6) where C is a constant so why TLE?

Segment Tree update 1 element in log(n), but you update the range from l to r, so it takes O(log(n) * (r — l + 1)) in 1 query, not log(n)

So you want to say that we can't solve it using segment tree, can we?

In my opinion, I think it can't.

yzkkai

I think you don't totally understand how segtree works.

Segtree is for optimizing "range" operation, and if you see your code again, you will find it only updates in the leaves, which means the segtree doesn't kick in.

Maybe you can change it to record every element's updation count to make segtree functional.

Sorry, my English is not good.

I know the concept of segTree and solved introductive problems by it. btw I'll try again thanks

Does someone have good blogs/material about square root decomposition technique for problems like F?

Minhazur

← Rev. 5 →

PROBLEM 4: shouldn't the output of the 5th test case be 4 ? aazz -> az az -> 2+2 ( SORRY I UNDERSTOOD IT WRONG )

tranluixuong

You can only split the original string into 2 strings with non-zero lengths. Given the input aazz, you can split the string to

a and azz
aa and zz
aaz and z.

There was an explanation for your concern in the contest's notification.

Happy coding ^^

Thanks . I just joined 20 minutes before the contest ended. I think I missed that notification.

OrientalMagician

Hi guys, I am confused. This is my first contest. Why does it show unrated on my profile? I thought I should get a rating as I am a newbie. Thanks

TheSky233

Every contest shows unrated before the ratings come out.

Shnatan

I have a doubt about my submission for F. My code is similar to the one provided in the editorial, the only difference being that I simply moved the iterator to the next element in the set after first setting the iterator to the lower bound of l. But it gives TLE on test case 18. To my knowledge, wouldn't it be better to just move the iterator to the next element in the set as it is already sorted rather than using a binary search to get the next valid index at every iteration? Here's my code -> https://mirror.codeforces.com/contest/1791/submission/192143264

Kev2103

Subject: Need Help for G2 In tutorial it is said we will need to do BS over all the values by taking it as our first problem. While solving I thought that if we took min out of all the starting costs it would be enough. In case of equality between 2 such values I took the one with greater cost from back. It is showing WA at test case 828 expected 2 output 1; what is wrong in my reasoning ?

Take a look at Ticket 16712 from CF Stress for a counter example.

Thanks a lot

jr3LOLO

it's good tutorial

meehirmanitrips

I participated in this contest but its not showing my profile in the official ranking, even though i am a newbie and my rating is below 1400 so i should be eligible. Can someone explain to me why its not showing my profile in official ranking.

islam15-14475

Why does this solution give the wrong answer? question (1791E) http://pastebin.ubuntu.com/p/TrRV4gSKG5/

Utkarsh_09

In problem E (Negative and positives), I write my comparator function for sorting, but why does this solution give runtime on test case 24? Can anyone explain? 1791E - Negatives and Positives 191930201

t_t_m

https://mirror.codeforces.com/blog/entry/70237

puspendra_09

Can anyone help in finding the test case in which my algorithm is failing in problem F. Range Update Point Query . submission Link

Take a look at Ticket 16717 from CF Stress for a counter example.

0rirobin0

Attention!

****My solution 191999829 for the problem 1791C significantly coincides with solutions 0rirobin0/191999829, rakat27/192015958 are not almost similar! We are not copied each other. ****Coincidentally our code ended up being structurally similar but the variables we took didn't match! here in 0rirobin0/191999829 i used test case variable as 't' and in rakat27/192015958 he took 'a' as a variable in test case!

****If you check the submission time you will see I submitted my code at first than rakat27 and i didnt copied him! please rejudge my proof and kindly bring my rating back! regards 0rirobin0 code screenshot link below ->

http://https://drive.google.com/file/d/1XmssHMy6TcCrNskv63Z1_cnT2Zj59ZAM/view?usp=sharing) https://drive.google.com/file/d/1N7xF8qEjwemqyukDQ1-6wXK42LkKLJpR/view?usp=share_link

WolG

Here is an alternative solution for problem G2 (Python): https://mirror.codeforces.com/contest/1791/submission/192216076

It is quite fast and in my opinion easier and faster than the original solution.

Obviously it is still nlog(n) because of the sort.

sumit21

I had done the same implementation also got accepted during contest but now it's showing tle in https://mirror.codeforces.com/contest/1791/submission/192033550. I don't know what's wrong with it. Please have a look

solutions gets judged with pretests before actual tests, only after the open hacking phase (12hrs) that they judge solutions with all testcases

set s; it= s.lower_bound(a); it= lower_bound(s.begin(),s.end(),a); whats the difference between two

sets and maps uses random access iterator and has their own lower_bound method that works in O(logN), but if you use lower_bound() on sets and maps, it works in linear time, meaning O(N)

lakshay05

Great Problem G2

satvik2323

In the problem G2, I am not getting how are they handling the case when the portal we are considering is included both times as the initial portal and in the minimum cost prefix. And in the tutorial's code what this piece of code is doing? int now = mid+1; if(mid > i) { price-=a[i].fr; now--; } can anyone help me out?

VigneshwaranMahadevan

So , i+1 here is the first index you went to from index 0 That is why , we subtract a[i].sc from c. Now , we have the prefix sums of costs which were sorted . So we do BS.

During BS, we are at index mid

if we are at mid what it means is Our first step was to index i+1 And we are currently at index mid (we could have come from 0 or n to mid)

mid could be > i or <=i

Now what happens to price alias pref[mid] when i < mid ? pref[mid] = sum of a[index].fr for index from 1 till index mid

But we already visited index i+1 (which is <=mid) . Because it was our first index! So we do not need to add a[i].fr again. Thats why we subtract a[i].fr from price if i < mid

Now coming to the number of teleporters you visit. If i >= mid: 1 2 ... mid-1 mid mid+1 .... i i+1 Your first visit was to index i+1 and then you visited all indices from 1 to mid In total 1 (index i) + mid(1 till mid) Thats why now is mid+1

Now what if i < mid?

1 2 ... i i+1 .. mid mid+1 ... You already visited index i+1 Now you visit till index mid — BUT SKIP INDEX i+1 So , you visit just mid number of indices But you initialised now to mid+1 So subtract 1 from now.

jerseyguy

still the index i should be included in the ans because it also comes under visited list but doing a now-- excludes that from the visited list???

hars_raj

Can someone help me figure out why this solution:192214278 for F 1791F - Range Update Point Query is giving WA. Thanks. Edit:- I was able to come up with a test case where the solution gives an unexpected answer:

Testcase

Take a look at Ticket 16716 from CF Stress for a counter example.

Cool!! but the code is giving the expected output for the provided test cases:

It doesn't, on IDEONE

One possible reason is undefined behavior in your code resulting in RTE across systems. (Looks like you are doing itr++ as part of loop increment after you have deleted/invalidated your iterator.

Yeah..I got it

__abs

Can someone help figure out why 192415650 (G2) gives WA?

1xx55

Why my code get TLE on test 18 for problem G1? I used qsort to sort the array. https://mirror.codeforces.com/contest/1791/submission/192238361

kamehamehaaa

Can someone help me figure out why 192601940 gives WA? https://mirror.codeforces.com/contest/1791/submission/192601940

musfiqurpavel

192566663

In problem F: why am I still getting TLE, despite keeping the how many times updated a[i]? Can someone explain it to me,

castlercodes

Can anyone explain why this code is failing this test case?

https://mirror.codeforces.com/contest/1791/submission/192817422

wn611

How to write the dp of the E question？

https://mirror.codeforces.com/blog/entry/112282?#comment-1000597

6 months ago, # ^ |

thank you!

ayush002024

love G2 .....!!

Slumio

Wow I'm seeing this editorial late

scrub_er

7 months ago, # |

G1 test case explanation is wrong

ARYANK3024

Can Someone help for which case i am getting the Wrong Answer in Problem 1791G2.

Here is my code link https://mirror.codeforces.com/contest/1791/submission/194376164

Cannabist_carlsen

I have a doubt in problem G1? Won't the answer depend on the index of the teleporter. For ex if the smallest teleporter placed at some index value which is too high and reaching there isn't possible i.e index of smallest teleporter > c coins given to us. But at the some time there exist a teleporter(not the smallest teleporter) whose { index + teleporting point <= c coins } . Is my question valid? Please help me out asap.

Viper07

3 months ago, # ^ |

211677653 this may help --> just sort adding both (index + Cost)

elvan

another easy way for easy version of teleporters 194870644

samulokito14

please mention your doubt, and talking of examples they are solved correctly! results of both examples are correct.

PoloBanker

i want to be a grandmaster

Ranjeetkumar06

6 months ago, # |

quetions NO.D where is mistake my code I don't know if you know or find mistake in my code please pin your massage ~~~~~ Your code here...

3 months ago, # |

can anyone please explain DP approach for Problem E

mason_24

2 months ago, # |

Please can someone explain to me what is the real goal on the problem G1. Teleporters (Easy Version). Even after reading the editorial for that problem. I still do not understand. Thank you.

BeanBurrito

The total cost of getting to a teleporter and using it (starting from point 0) is its location plus its cost (a_i + i).

Therefore, if you sort all the teleporters according to this value, you can ensure that you will always pick the teleporter that is the cheapest to get to and use, and maximize the number of teleporters that you use.

vishnupriya1

5 weeks ago, # |

https://mirror.codeforces.com/contest/1791/submission/220502229 anyone plz let me know whats wrong with my code

flamestorm's blog

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