Educational Codeforces Round 162 Editorial

#	User	Rating
1	tourist	4009
2	jiangly	3823
3	Benq	3738
4	Radewoosh	3633
5	jqdai0815	3620
6	orzdevinwang	3529
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8	Um_nik	3396
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#	User	Contrib.
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3	maomao90	162
3	atcoder_official	162
5	adamant	159
6	-is-this-fft-	158
7	awoo	157
8	TheScrasse	154
9	Dominater069	153
9	nor	153

1923A - Moving Chips

Tutorial

Solution (BledDest)

t = int(input())
for i in range(t):
    n = int(input())
    a = list(map(int, input().split()))
    l, r = a.index(1), n - a[::-1].index(1) - 1
    c = a.count(1)
    print(r - l - c + 1)

1923B - Monsters Attack!

Idea: BledDest

Tutorial

Solution (Neon)

#include <bits/stdc++.h>
 
using namespace std;
 
int main() {
  int t;
  cin >> t;
  while (t--) {
    int n, k;
    cin >> n >> k;
    vector<int> a(n), x(n);
    for (auto& it : a) cin >> it;
    for (auto& it : x) cin >> it;
    vector<long long> s(n + 1);
    for (int i = 0; i < n; ++i) s[abs(x[i])] += a[i];
    bool ok = true;
    long long lft = 0;
    for (int i = 1; i <= n; ++i) {
      lft += k - s[i];
      ok &= (lft >= 0);
    }
    cout << (ok ? "YES" : "NO") << '\n';
  }
}

1923C - Find B

Idea: Roms

Tutorial

Solution (Roms)

#include <bits/stdc++.h>
 
using namespace std;
 
const int N = 300043;
 
int t;
int n, m;
int a[N];
long long sum[N];
int cnt1[N];
 
int main() {
    ios_base::sync_with_stdio(false);
    
    cin >> t;
    for (int tc = 0; tc < t; ++tc) {
        cin >> n >> m;
        memset(sum, 0, sizeof(sum[0]) * (n + 5));
        memset(cnt1, 0, sizeof(cnt1[0]) * (n + 5));
        for (int i = 0; i < n; ++i) {
            cin >> a[i];
            sum[i + 1] = sum[i] + a[i];
            cnt1[i + 1] = cnt1[i] + (a[i] == 1);
        }
        
        for (int i = 0; i < m; ++i) {
            int l, r;
            cin >> l >> r;
            --l;
            int cur_cnt1 = cnt1[r] - cnt1[l];
            long long cur_sum = sum[r] - sum[l];
            
            if((r - l) + cur_cnt1 <= cur_sum && r - l > 1)
                cout << "YES\n";
            else
                cout << "NO\n";
        }
    }
    return 0;
}

1923D - Slimes

Idea: BledDest

Tutorial

Solution (Neon)

#include <bits/stdc++.h>
 
using namespace std;
 
int main() {
  int t;
  cin >> t;
  while (t--) {
    int n;
    cin >> n;
    vector<int> a(n);
    for (auto& x : a) cin >> x;
    vector<int> ans(n, n);
    for (int z = 0; z < 2; ++z) {
      vector<long long> s(n + 1);
      for (int i = 0; i < n; ++i) s[i + 1] = s[i] + a[i];
      vector<int> p(n, -1);
      for (int i = 1; i < n; ++i) {
        int j = (z ? n - i - 1 : i);
        int l = 1, r = i;
        while (l <= r) {
          int m = (l + r) / 2;
          if (s[i] - s[i - m] > a[i] && p[i - 1] >= i - m) {
            ans[j] = min(ans[j], m);
            r = m - 1;
          } else {
            l = m + 1;
          }
        }
        if (a[i - 1] > a[i]) ans[j] = 1;
        p[i] = (a[i] == a[i - 1] ? p[i - 1] : i - 1);
      }
      reverse(a.begin(), a.end());
    }
    for (int i = 0; i < n; ++i)
      cout << (ans[i] == n ? -1 : ans[i]) << ' ';
    cout << '\n';
  }
}

1923E - Count Paths

Idea: BledDest

Tutorial

Solution (awoo)

#include <bits/stdc++.h>
 
#define forn(i, n) for (int i = 0; i < int(n); i++)
 
using namespace std;
 
int n;
vector<int> a;
vector<vector<int>> g;
 
long long ans;
vector<map<int, int>> cnt;
 
void dfs(int v, int p = -1){
    int bst = -1;
    for (int u : g[v]) if (u != p){
        dfs(u, v);
        if (bst == -1 || cnt[bst].size() < cnt[u].size())
            bst = u;
    }
    for (int u : g[v]) if (u != p && u != bst){
        for (auto it : cnt[u]){
            int x = it.first, y = it.second;
            if (x != a[v]) ans += cnt[bst][x] * 1ll * y;
            cnt[bst][x] += y;
        }
    }
    if (bst != -1) swap(cnt[bst], cnt[v]);
    ans += cnt[v][a[v]];
    cnt[v][a[v]] = 1;
}
 
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int t;
    cin >> t;
    while (t--){
        int n;
        cin >> n;
        a.resize(n);
        forn(i, n) cin >> a[i];
        g.assign(n, {});
        forn(_, n - 1){
            int v, u;
            cin >> v >> u;
            --v, --u;
            g[v].push_back(u);
            g[u].push_back(v);
        }
        ans = 0;
        cnt.assign(n, {});
        dfs(0);
        cout << ans << '\n';
    }
    return 0;
}

1923F - Shrink-Reverse

Idea: adedalic

Tutorial

Solution (adedalic)

#include<bits/stdc++.h>
 
using namespace std;
 
#define fore(i, l, r) for(int i = int(l); i < int(r); i++)
#define sz(a) int((a).size())
#define all(a) (a).begin(), (a).end()
 
typedef long long li;
typedef pair<int, int> pt;
 
const int INF = int(1e9);
const int MOD = int(1e9) + 7;
 
int add(int a, int b) {
    a += b;
    if (a >= MOD)
        a -= MOD;
    return a;
}
int mul(int a, int b) {
    return int(a * 1ll * b % MOD);
}
 
namespace SuffixArray {
    string s;
    vector< array<int, 2> > classes;
    
    vector<int> build(const string &bs) {
        s = bs;
        s += '$';
        
        vector<int> c(all(s)), ord(sz(s));
        iota(all(ord), 0);
        classes.resize(sz(s));
 
        for (int len = 1; len < 2 * sz(s); len <<= 1) {
            int half = len >> 1;
            fore (i, 0, sz(s))
                classes[i] = {c[i], c[(i + half) % sz(s)]};
            
            sort(all(ord), [&](int i, int j) {
                return classes[i] < classes[j];
            });
            c[ord[0]] = 0;
            fore (i, 1, sz(ord))
                c[ord[i]] = c[ord[i - 1]] + (classes[ord[i - 1]] != classes[ord[i]]);
        }
        
        c.pop_back();
        for (int &cc : c)
            cc--;
        return c;
    }
};
 
int n, k;
string s;
 
inline bool read() {
    if(!(cin >> n >> k))
        return false;
    cin >> s;
    return true;
}
 
string calcZero(string s) {
    int rem = k;
    int pos = 0;
    for (int i = sz(s) - 1; rem > 0 && i >= 0; i--) {
        if (s[i] == '1')
            continue;
        
        while (pos < sz(s) && s[pos] == '0')
            pos++;
        if (pos >= i)
            break;
        
        swap(s[pos], s[i]);
        rem--;
    }
    return s.substr(s.find('1'));
}
 
string calcOne(string s) {
    reverse(all(s));
    
    auto c = SuffixArray::build(s);
    int cntOnes = count(all(s), '1');
    
    array<int, 3> mn = { 2 * sz(s), INF, -1 };
        
    int u = 0, curOnes = 0;
    fore (i, 0, n) {
        while (u < sz(s) && (u - i < cntOnes || cntOnes - curOnes > k - 1)) {
            curOnes += s[u] == '1';
            u++;
        }
        if (u - i < cntOnes || cntOnes - curOnes > k - 1)
            break;
        
        array<int, 3> curAns = { u - i, c[i], i };
        mn = min(mn, curAns);
        
        curOnes -= s[i] == '1';
    }
    assert(mn[2] >= 0);
    
    string res = s.substr(mn[2], mn[0]);
    int toAdd = cntOnes - count(all(res), '1');
    for (int i = sz(res) - 1; toAdd > 0 && i >= 0; i--) {
        if (res[i] == '0') {
            res[i] = '1';
            toAdd--;
        }
    }
    return res;
}
 
inline void solve() {
    auto s1 = calcZero(s);
    auto s2 = calcOne(s);
 
    if (sz(s1) > sz(s2) || (sz(s1) == sz(s2) && s1 > s2))
        swap(s1, s2);
    
    int res = 0;
    for (char c : s1)
        res = add(mul(res, 2), c - '0');
    cout << res << endl;
}
 
int main() {
#ifdef _DEBUG
    freopen("input.txt", "r", stdin);
    int tt = clock();
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    cout << fixed << setprecision(15);
    
    if(read()) {
        solve();
        
#ifdef _DEBUG
        cerr << "TIME = " << clock() - tt << endl;
        tt = clock();
#endif
    }
    return 0;
}

def f(c,l,r): if l == r: return False return c[r]-c[l-1] >= 0 def solve(): n,q = list(map(int,input().split())) a = list(map(int,input().split())) b = [ -1 if x == 1 else x - 1 for x in a] c = [0] for i in range(n): c.append(c[-1]+b[i]) for i in range(q): l,r = map(int,input().split()) if f(c, l, r): print('yes') else: print('no') t = int(input()) for _ in range(t): solve()

void dfs(int s, int p, vector<int> *adj, map<int,int> &m, ll &ans, vector<int> &a){ ans += m[a[s]]; int t = m[a[s]]; for(int u:adj[s]){ if(u==p) continue; m[a[s]]=1; dfs(u,s,adj,m,ans,a); } m[a[s]]=t+1; } // not pasting template here, scan -> cin, print -> cout void solve() { int n; scan(n); vector<int> a(n); vscan(a); vector<int> adj[n]; for(int i=0; i<n-1; i++){ int x,y; cin>>x>>y; adj[x-1].push_back(y-1); adj[y-1].push_back(x-1); } map<int,int> m; ll ans = 0; dfs(0,-1,adj,m,ans,a); print(ans); return; }

Comments (84)

Write comment?

Yugandhar_Master

9 months ago, # |

I think E has same idea as this

→ Reply

bruce_x001

Problem C video Editorial : YOUTUBE VIDEO LINK Audio : English

rachitguptadev

9 months ago, # ^ |

← Rev. 2 →

-35

Bro in problem C why we can not only check the no. of ones in subarray, if no. of one is greater than (r-l)/2 than not possible . why my solution fail can you tell me. Pease Don't downvote me.

← Rev. 4 →

Test case :

6 1

1 1 1 1 4 4

1 6

You can choose array b as : 2 2 2 2 2 2

The main thing is ... You have to place 1 (in array b) on the elements which are not equal to 1 (in array a). and After placing 1, the remaining sum should be distributed among the elements which are 1 in array a. I hope it is clear

You can refer to the video solution for detailed explanation.

I got it

Physical

+13

bro sent your code as a spoiler or as a link

Bro why is so dowvnvotes in my question

Because of the way you wrote the code in your comment, the first few lines have a very large font size , and the rest are very small with no ne lines for each statement , it's not readable and it takes up a lot of space.it's a common practice to put your code in a spoiler

Milan

+104

E can be solved linearly.

edit : I lost some rating but I think the problems were very nice :)

cp_boy

It was very beautiful solution.

peek4b00

Can you explain this solution a bit ,like what is going in the dfs call

+15

At any point in the dfs, we keep track of how many beautiful paths we can create if we encounter every given color (the array stores it). So when we enter a vertex in the dfs, we can increment the beautiful path counter by the value for the color in the array.

Then, on each subtree we explore from this new call to dfs, we just need to reset the value of the current color to 1, because it will block any path to previously visited vertices of the same color.

Finally, when leaving the dfs, we should increase the initial value of the current color by one, because there will be one more beautiful path if we encounter the same color again. Try drawing what happens, it helps.

Thank you for the explanation. Indeed a very beautiful solution.

Akil_Newaz_Chy

2 months ago, # ^ |

can you explain it a little more , please! specially the 3rd paragraph!

SergiiGolovko

Outstanding solution, thank you for sharing!

MuhammadSawalhy

-12

Wow, exactly like my solution: 248115690. The human brain is mysterious!

asdvgy

8 months ago, # ^ |

So elegant!

zc2707

Wow, I just see a solution of E which takes $$$O(n)$$$ time. It just uses 1 DFS to calculate the result synchronously just by how we will do to find out the result of one color.

PsychoPinkQ

← Rev. 5 →

I solved E with centroid decomposition (which is $$$O(n log n)$$$) but got TLE.247977712

But the Editorial says authors were expecting $$$O(n log^2 n)$$$ solutions. I'm confused.

iiand

Are you sure it is $$$O(n log n)$$$? It can't pass even if you increase time limit to $$$15$$$ seconds.

← Rev. 13 →

I check it again, I think all the things I do in function CD is linear.

$$$T(n) = 2T(\frac{n}{2})+O(12 n)$$$(worst case of recurrance) implies that my solution has a time complexity of $$$O(n log n)$$$(Master theorem). Maybe I implement something wrong.(Or learnt centroid decomposition or Master theorem in the wrong way?)

Btw, how did you know my code would TLE even if the time limit was 15 seconds?

← Rev. 3 →

You can create a mashup and increase TL to 15 seconds. I don't know centroid decomposition lol:(

I checked again. It works $$$8$$$ seconds on test where $$$t = 1, n = 20000, a[i] = 1$$$ and graph edges = $$$(1, 2), (2, 3), (3, 4)$$$, .... So i think it's $$$n^2$$$.

+11

Yes, you are probably right, my solution should be $$$O(n log n)$$$ indeed. However, the way i find centroids is wrong, causing my solution having $$$O(n^2)$$$ complexity. I fixed that and got AC.

Thanks a lot!

s26i_kulaev

Hello Codeforces! Please explain what virtual trees are (they are used in the second solution of the problem E).

User0069

https://mirror.codeforces.com/blog/entry/76955

ahmadrafi

Need Help.

Why does this solution give tle while the one given in editorial passes for the problem 1923E - Count Paths.

248160733

el_tunel

It's probably because you are merging everything into the parent instead of merging smaller children into the largest child. Merging everything means more merging work, which takes more time.

aalokpatel

Can someone please tell on which case this is failing?

Spoiler

void solve() {

ll n,q;

cin>>n>>q;
ll arr[n];
for(ll i=0;i<n;i++){
    cin>>arr[i];
}
ll pre[n];pre[0]=arr[0];
for(ll i=1;i<n;i++){
    pre[i]=pre[i-1]+arr[i];
}
while(q--)
{
    ll l,r;
    cin>>l>>r;
    ll sum = pre[r-1];
    if(l>1){
        sum -= pre[l-2];
    }
    if(l==r){
        cout<<"NO\n";
    }
    else{
        ll length = (r-l+1);
        ll atleast = sum/length;
        if(atleast>1){
            cout<<"YES\n";
        }
        else if(sum==3 && length==2){
            cout<<"YES\n";
        }
        else{
            ll total_ones = length - (sum%length);
            if(total_ones>(length/2)){
                cout<<"NO\n";
            }
            else{
                cout<<"YES\n";
            }
        }
    } 
}

}

Bro sent your code as a link or as a spoiler

nguyenquocthao00

7 months ago, # ^ |

case 1 1 1 3 => expected NO, you answer YES
case 1 1 2 2 => expected YES, you answer YES

H2J

I had an idea that works in O(n):

Check this code:

https://mirror.codeforces.com/contest/1923/submission/247999826

dragoonSlayer

i have solved E linearly. Here is the solution

here ans[c] means how many visited nodes of color c are available to be the first vertex .

when we are going forward we can only add 1 node of that color with new node. And when we go backward we are getting 1 extra node to add with new nodes.

AnishchukT

That's so elegant!

yeetholmes619

Lovely solution! very elegant. I solved it in O(n) as well, but not nearly as beautiful. 248719267

Kanishk_0012

In problem B, why is it always optimal to hit the nearest monster? There can be a possibility that there was another monster far away with much much greater health, and if certain bullets were not provided to them within the starting seconds, they would definitely hit the origin.

If that's the case, then you are doomed, since if you don't kill the closer monster first he will kill you before the further monster reaches you.

kojiro

I would love to know if someone can tell me why my solution to problem C is so slow (2308 ms)

Jerryliu2008

I think it's because you used endl instead of '\n' at the end of outputting a new line. I don't quite know how this works exactly but endl does a operation called "flushing" after it outputs while '\n' doesn't. You might also want to add the following to improve your IO speed (This will only work with \n, endl will still slow your IO speed even if you add this code):

ios_base::sync_with_stdio(false);
cin.tie(nullptr);

Here is my submission with endl and this code (1185 ms): submission

Here is my submission with '\n' and this code (296 ms): submission

For further information on how this works read this USACO guide article: Link

Thank you for your comment but I have defined endl as "\n"! Ill' add those 2 lines, thanks!

pavangadhiya06

yes, that's correct i also getting reduced time with 186ms from 982ms but just writing those to lines...

HideBeyondYou

The third case in Tutorial-C: If this sum is greater than ...

Change 'greater' to 'smaller'.

0x0002

You can use divide and conquer on tree to solve problem E easily in $$$O(n \log n)$$$ too. It is easy to think.

tminh_hk20

yeah, just use centroid decomposition. But I use an unordered set and could AC it. How can you do it in Nlogn?

If someone want to consult a centroid decomposition + set solution can see my submission: 248033502. My idea: I can count the number of way cross the centroid node for all node and use set to manage the first node in DFS way has color C.

Nightmare_luffy

I am unable to understand why the solution to problem E has time complexity O(n log^2 n ). For every node, we are iteration through all the children, and then for every child we are iterating through every color it has. Can someone explain it to me?

xcx0902

It's a type of "DSU Merging". When a vertice's color set is iterated, the destination set size must be at least twice of the original one, so the total movement is at most $$$O(\log n)$$$ for each node, then the final movement is at most $$$O(n \log n)$$$. With the complexity of map, the final complexity is $$$O(n \log^2 n)$$$.

Thanks! btw, I found this if anyone is looking for a detailed explanation.

Shivam2105

I think there are some mistakes in tutorial for problem 1923C — Find B

To answer query l , r at first let's try to find the array b with the minimum value of ∑bi . For this, for indexes i , where ci=1 we have to set bi=2 , and for indexes i , where ci>i we have to set bi=1 . Thus, the sum of all elements in array b equal to cnt1⋅2+(r−l+1)−cnt1 or (r−l+1)+cnt1

. Now we have three cases:

If this sum is greater than ∑ri=lci then the answer is NO

If this sum is equal to ∑ri=lci then the answer is YES

If this sum is greater than ∑ri=lci then we can add this excess to the maximal element in array b In this case, the answer is YES

instead of ci>i, shouldn't it be ci>1 ?

In third case, shouldn't it be smaller instead of greater?

gsomani

My solution for C got hacked. It seems somes python solutions are just taking more time. Can somebody explain ?

233L

About F's fact 4, two binary strings should have same number of '1'. Is it true?

sahaun

If $$$s1=s2$$$, there will be same number of $$$1$$$'s. Otherwise, $$$s1<s2$$$ or $$$s2<s1$$$, and they don't have the same number of $$$1$$$'s. The same relationship will hold between $$$s1'$$$ and $$$s2'$$$ since the fact mentions that we are replacing the same number of $$$0$$$'s with $$$1$$$'s in both strings.

For this problem though, $$$s1'$$$ and $$$s2'$$$ will always contain the same number of $$$1$$$'s. But this condition is not necessary for $$$s1$$$ and $$$s2$$$.

Thanks!

1BeeNY1

I solved E using stack for each color.

Here is my solution: 248216745

sameer1720034

Can someone tell me why my solution for B is not working.

My Submission

Approach — stored the sorted vector (distance,hp) in vector p. Then,iterated through vector p and for each monster, calculated the time it would need to be killed by the forumla hp/k. also calculated bullets left in extra. at the start of each iteration , subtracted extra bullets

nayanrathore

D was a realy good question. Great Contest

Isaac_Newton__

248285844 Problem D Whats wrong with my code please help. I just don't get why am i getting wrong answer.

ustaad_bismillah

Explaining Tourist's Solution to D Problem

Objective: For each slime, find min no of operations to eat that slime. One Operation is eating of one slime by an adjacent bigger slime.

Some observations:

For each slime, if there is a bigger slime to immediate left/right, then ans for this slime = 1.

Now in other cases

If there is a consecutive set of equal-sized slimes, they can't do any operation on each other.
Any merging operation to make a bigger slime are made either to the LEFT or RIGHT, not including the index of the slime.
Looking at one side (left or right), there must be at least 2 different-sized slimes on this side.
Since eventually the adjacent slime needs to get bigger than the current slime, we need to look for the min slimes to include on one side, which has its prefix sum > current slime size, and no of distinct slimes > 1.
Since the prefix sum is monotonous for a contiguous range, we can perform binary search on both side to find the min number of slimes to include to satisfy the condition.
Explanation of setting limits for binary search: LEFT keep the lo limit at 0, and hi limit at the largest index where slime size is not equal to that of the slime to the immediate left of current slime. Because need to ensure at least 2 different-sized slimes be included in the range.

Pre-processing:

l(n) and r(n) : Store the begin and end index of the contiguous equal-sized slime windows.
p(n+1) : prefix sum array : its monotonicity property helps use binary search.

Overall Time Complexity: O(nlogn)

O(n) time for preprocessing
O(nlogn) time for the core implementation: logn time to binary search to LEFT and RIGHT of the current index.

248285844 Problem D Whats wrong with my code please someone help. I just don't get why am i getting wrong answer.

Try checking the ok condition before starting the binary search. Sorry , you have already done it. I juat saw it

I found the mistake. I was using return type 'int' for 'summ', but shouldn't it give integer overflow for this?

Sms0000

For Problem E, I came up with this simple solution.

Nice solution, you can replace your map with an array for a better complexity.

Oh yes! Thanks!

not_wiz

Would you like to explain ?

Whenever you visit a node s, a[s] is the value of that node. m[a[s]] will take count of how many nodes have value a[s] before this node with which we can make pair, I mean no other a[s] comes in a way.

Suppose you visit s, than for a node lying in its subtree there is no other way to make pair instead of s, because s comes in between any other s. So making m[a[s]]=1

While coming out of the highest level s increasing the number of s by 1 for next s.

rmz

can we solve C using sparse table ?

pranshu_2004

Can anybody tell , for problem 2 why this code is giving error on test case 2 , https://mirror.codeforces.com/contest/1923/submission/248233287

CM_This_Year

For problem D : Can anyone tell why is this code giving wrong answer??248345962

nichke

Can someone explain why jiangly's solution to problem E works in O(Nlog^2N). I know it utilizes small to large merging, but why does merging by the size of color sets yield that complexity? When we merge some node's children, set size won't necessarily go up 2x. For example combining sets (1,2,3) and (1,2,3) will give us (1,2,3) again, so the regular proof does not apply here.

Edit: Nvm, I neglected the fact that number of colors is always less or equal than the number of nodes in a subtree. The proof is trivial.

Minerva2005

How does one come up with solutions like that of problem C? :/

alexiuscp

The goal is to make the elements different from the original, whilst not changing the sum.

An easy thing to do, is decrement one number and increment another. By doing this, we've changed both the numbers from the original whilst still having the same sum.

Now obviously, we cannot decrement 1, because the third rule states any number must be > 0. Therefore, we are forced to increment all 1s, and forced to decrement all numbers > 1.

It's easy to see that this is only possible, if the total of all numbers > 1 is not less than the total of numbers equal to 1.

We can then, of course answer the range queries using prefix/suffix sum.

sarvagya2545

I do not understand why does my submission not pass the second test case and what am I doing wrong here. Can someone please explain? 248388078

adaptatron

I made video editorial for E discussing the DP idea (excluding the small to large trick).

I also made a contest with easy version of problem E and some followup problems. Access it here

Plz_Accept

Thanks

Lavine

Problem F's "Fact 4",You should specify that s'1 and s'2 have the same number of 1s , otherwise your statement is wrong.

s1=1000011 s2=1000100 k-1=2 s'1=1001111 s'2=1000111 s1 < s2 and s'1 > s'2

adedalic

Done

mircea85

For E with Virtual Trees — I think you can do just an modified Euler walk of the tree first, and then walk on only the nodes of the same color in the order of the Euler walk: it will be eqv. to doing DFS on the Virtual Tree. So should be easy to do DP in each node for all its top-level children with/without being a direct descendent of the parent.

The final complexity should be just O(n), better than the O(n log n) of the tutorial.

The modification of the walk is that you put the node each time you visit it, not only first & last time.

Kageyama_29

can someone give me the testcase where this solution (https://mirror.codeforces.com/contest/1923/submission/248724413)

panyi

In problem D,my code got a "Wrong Answer" on test 2. Can anyone give me a hack? Many thanks.

https://mirror.codeforces.com/contest/1923/submission/248850524

Sandeep_9198

Problem C : what's the mistake in this code? Please help. https://mirror.codeforces.com/contest/1923/submission/249311392

kaashmonee

Is there a typo for the editorial for problem C? Should it be, "and for indexes $$$i$$$, where $$$c_i > 1$$$" instead of $$$c_i > i$$$? That makes more sense to me.

Rafiullah

7 months ago, # |

How can we do B using DP?

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