-Morass-'s blog

Problem Topics
By -Morass-, history, 7 years ago, In English

Good Day to you!

I've been asked to make some topic-wise list of problems I've solved. Even though I couldn't involve all problems, I've tried to involve at least "few" problems at each topic I thought up (I'm sorry if I forgot about something "easy"). I've alredy made such list once anyway I've tried to include more problems now — so here it is:

aho
automat
belman-ford
bfs
bfs-grid
big
binary_search
bits
bitset
bridges
brute-force
centroid
coloring
combinatorics
constructive
dfs
digits
dijkstra
divide_conquer
divisors
dp
dsu
euler_function
euler_tour
factorization
fenwick
fft
flow
flow-matching-like
floyd-warshall
friedvaldAlgorithm
game_theory
gauss
geometry
graph
greedy
hash
hull
chess
implementation
inclusion-exclusion
interactive
isomorphism
josephus
KMP
lca
lcs_subsequence
lct
lis
matching
matrix
matrix_exponentiation
mcmf
median
meet_in_middle
MO
next
np-hard
number_rectangle
number_theory
observation
oeis
offline
palindromes
patter-matching
permutations
persistent_segment_tree
preprocess
prime-count
prime-testing
probability
recursion
RMQ
rope
scc
segment_tree
sequences
sieve
simulation
sorting
spanning_tree
spfa
sqrt
stl
strings
suffix_array
ternary_search
topo
treap
tree
tree-dp
trie_bit
trie_string
TSP
two-pointers
wavelet_tree
Zfunction
2SAT

Finally if you would like to add some problem to the list — even though I would be glad, please do so only in case of:

  1. It is very interesting

  2. There is nothing, or low number of problems in the topic

  3. You add it in "bigger amount" at once

Thank you.

Offcourse if you have any remarks, questionns or requests, don't hesitate to ask.

PS: I'm sorry but there might be some duplicities. In that case, either report it or ignore it (unless they are in different topics, then it have reason :) )

Good Luck & Have Nice Day

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Comments Comments (119)
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7 years ago, # |
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Thank You :)

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7 years ago, # |
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http://www.spoj.com/problems/PAIRDIV/ (6) //cyka möbius -_-

It seems that you don't like Möbius inversion much.

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    7 years ago, # ^ |
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    Hello, Yes,

    I'm not mathematician and it seems slightly like magic to me :'( I find it hard somehow :/

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      6 years ago, # ^ |
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      Good day to You too, I'm sorry to ask such a question, but I really couldn't really find where I can submit problem called "8015 — Alice and Bob play Contact" in a division trie_string, because when I go to UVA and insert a value 8015 in a problem id in a Quick Submit, I get a message: "The selected problem ID does not exist.", what is its actual id by which I can submit the problem?

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    4 months ago, # ^ |
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    Hello there! I know it has been a very long time but I was trying to solve this problem PAIRDIV, and I couldn't solve it using mobius inversion. I managed to solve it in another way though, but I would like to know how to solve it with mobius inversion. Can you help me, please?

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7 years ago, # |
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Are the numbers in the parenthesis your judgement of how difficult each problem is?

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    7 years ago, # ^ |
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    Good day to you,

    yes, exactly as you say. But as it is stated — it is my opinion so it might be "a little bit" off :)

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7 years ago, # |
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I find it funny that there is a topic "oeis". xD

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    7 years ago, # ^ |
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    Good day to you,

    well right :D .. it is mostly math.

    It is might be used slightly in "different" manner then normal math problems. I.e. you can brute-force small test-cases only and then "google the rest" .. So math but basically different approach ^_^

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7 years ago, # |
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Can you please tell what are the problems from LA are?? i mean what is LA?? i have never heard of it.

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    7 years ago, # ^ |
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    Good day to you,

    sure: LA stands for Live Archive — it is a judge which stores most of the problems from Regional Contests + World Finals

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      7 years ago, # ^ |
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      Ok... never came across the acronym before :).Thnx for the prompt reply.

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7 years ago, # |
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what is "big"?

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    7 years ago, # ^ |
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    Good day to you,

    This topic stands for Big Integer — so problems with numbers which doesn't fit in 264 :)

    Have Nice Day ^-^

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7 years ago, # |
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I've got a nice problem for tree-dp

Torque and edges

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7 years ago, # |
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are all the questions tough or are some questions easy, how does a pupil like me approach those questions??? I simply want to ask that how do i use this resource for optimum benefit!!!

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    7 years ago, # ^ |
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    Good day to you,

    imho there shall be easy questions too. Some of them are marked by a number (by which I've tried to estimate the difficulty). So you might try (firstly) problems marked with lower numbers (lets say lesser/equal 3 or 4)

    Wish you a nice day,

    ~/Morass

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      7 years ago, # ^ |
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      thank you for such a quick reply. is the estimate out of 10.

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        7 years ago, # ^ |
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        Yes it is out of 10 (even thought — I think I've never used such big number :P) [but again.. it is just estimation, so might not be absolutely correct :) ]

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          7 years ago, # ^ |
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          can you give an estimate that what will be my rating like if I solve questions till level a)5 b)6 c)7 d)8(will take a long time to reach here.)

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            7 years ago, # ^ |
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            Unfortunatelly it is not much possible imho :'(

            Firstly, even person with lover rating can solve hard problem. It will just take him more time to come-up with solution and/or to code the solution.

            Secondly some problems fit to some people more — so it varies.

            Another thing is that many problems here are algorithmic. So sometime the hardest part might be algorithm itself. It indeed might be hard to code — and even more difficult to come with. On the other hand, it is not that hard to find such algorithm somewhere on google, learn it, code it.. and then it colsts "nothing" when you use it for the second time (even thought It costed many hours for the first time).

            So in my opinion, I can't say it... Anyway if you would be able to do that fast (so it would easily fit into codeforces contest) the rating might possibly probabli be somewhere between purpble to red (the range you told)

            Also note, that codeforces stile problems might be different from "ACM"-stlye problems... and also from some direct-method SPOJ problems.

            So well, sorry this didn't help you much, but that is my opinion :)

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              7 years ago, # ^ |
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              No No, infact I WANTED an answer to my silly question but you gave me the answer I NEEDED to progress further, thank you so much!!

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7 years ago, # |
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Sir, what do you think about the book — competitive programming 3 -by steven halim and felix halim? If I solve all the problems mentioned in that book will I improve? If yes, then how much?

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    7 years ago, # ^ |
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    Good day to you,

    Well firstly, I've not read the book so not sure if my answer will be "valid". Anyway I know some people who did so (and I've also heard a little bit about it and so on):

    The book introduces most of the algorithms which are considered "basic" so imho it is "almost necessary" to know these algorithms as "ground" for almost any (at least a little bit) advanced skill-level. I heard it is a great book (the algorithms are well described there) so it is worth giving it a try (well you shall learn the algorithm somewhere — so why not from here?). Anyway what I also think is, that it won't ensure you any of the skill-levels: There are many more thinks, which "shuffled" together makes you good at CP and the knowledge of algorithms is "only" (but it is imho important) one of the "carrier pillars".

    Again, I haven't read it and this is just my option :)

    Wish you a nice day ^_^

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      7 years ago, # ^ |
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      This one is nice for Z function . https://www.codechef.com/problems/CHSTR

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        7 years ago, # ^ |
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        Good day to you

        Thank you very much for suggestion, anyway sadly I'm not able to edit my blog anymore to add it :'(

        Seems I'm getting "504 Gateway Time-out" every time I try so and I'm unable to resolve this problem .. sorry :'(

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          7 years ago, # ^ |
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          Its ok. Thank you for the wonderful list :)

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7 years ago, # |
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840D - Destiny is a nice example of a tricky wavelet tree.

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7 years ago, # |
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842D - Vitya and Strange Lesson is another trie_bit problem it might be a good addition since the trie_bit list is pretty small.

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    7 years ago, # ^ |
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    Good day to you sahil070197 GreenGrape usernameson,

    Thank you for your contriution. Sadly I can't update the blog anymore (due to "504 Gateway Time-out") :'(

    If I would miraculously evade it one day, I'll add those problems,

    Good Luck & Have Nice Day!

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7 years ago, # |
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It would be the greatest help i have ever had on CODEFORCES if you added articles for each type of problems. Because after learning the type of algorithm, the problem list you wrote is the best to deepen it. Anyway thanks a lot BRO!!!.

by the way your post is already is the best profit . . .:)

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7 years ago, # |
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Hello! Why do you have two "Zfunction" tags? Do they serve different purpose?

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    7 years ago, # ^ |
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    Good day to you,

    nope, seems to be mistake, thank you :)

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7 years ago, # |
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Add this to geometry: https://icpc.kattis.com/problems/airport Really nice problem :)

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7 years ago, # |
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Aho-Corasick: http://mirror.codeforces.com/problemset/problem/346/B

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7 years ago, # |
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Auto comment: topic has been updated by -Morass- (previous revision, new revision, compare).

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    7 years ago, # ^ |
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    Congrats, I see you managed to get around 504-Gateway Time-out.

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      7 years ago, # ^ |
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      Thanx ... I was trying for more than week and now it magicaly worked .. I was really surprised :)

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7 years ago, # |
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Thank you very much, very interesting:))

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7 years ago, # |
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Nice list! Thanks

I also use the Categories section on ahmed_aly's A2 online judge.

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7 years ago, # |
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Auto comment: topic has been updated by -Morass- (previous revision, new revision, compare).

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7 years ago, # |
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Hello sir, I am doing CP for over a year now(though not on CF) and have learnt basic algorithms(those taught in 6.006 class MIT). What according to you should I do to improve my skills?

EDIT:I forgot to mention that nowadays I am giving contests and reading blogs of people on CF and topcoder but I cannot get 100% of what author is trying to say(in editorial or blog):(

Is there anything I can do or should not do at this point of time?

Thank you.

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    7 years ago, # ^ |
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    Good day to you,

    Firstly, this is HARD KIND of question, since it is not directly on CP but slightly on "psychology"/"learning" which might be different for different people (so question is whether programmers are the right people to answer this question). Also, as you can see, my progress is not that good so maybe it would be better to ask some red-coder (or nutela).

    Here is the text I typically use for such kind of question so hope you'll find something in it:

    In first place, one need obtain knowledge in basic algorithms: There are many algorithms which are not hard, yet it is hard to do even medium (sometime easy) problems without them. Starting from dfs/bfs/sieve/graph-algos/sorting/....(many many other). So find some way to learn them. Usually, find some good blod (codeforces/geeksforgeeks/some school's lectures/so on..).

    While doing the above, one has to catch some coding/debugging concepts. Afterward it is imho good to do many easy (at most medium) problems to improve coding skills. Even though many people undeestimate this, it is very important to get to phase where you can code what you know (well, it might sound stupid, but many times one know a solution one hour before end of contest, yet he ends coding 10 minutes before end when he starts his 20 minute debugging phase [and both could be significantly reduced])

    As one improves (hopefully), he must start doing harder and harder problems and soon with the harder problems, he must lear also advanced algorithms: Suffix Array/HLD/Segment Trees/...(and many many others) which are usually not "that" necessary for easier problems.

    Also during all phases, it is good (even thougt one spends a lot of time by coding) spend some time by reading:

    a) New algorithms helpful things b) Editorials for what one doesn't know (firstly THINK about the problems and if nothing come, search for solution. Sometime one just find he's "stupid" but many times one discovers "new amazing" techniques)

    Also sometimes it is interesting to peek to solution of others... even (or maybe BEST) after you solve the problem. Sometimes there is much better solution then you came with, sometimes there is something awesome (like algorithmic/or/language trick) which might simplify your futher coding.

    Also sometimes it is good to "measure twice, cut once"... thinking for a while even if you know the solution. Sometime you find improvement, or reduce it by redundant part..

    anyway... solve solve solve ~ that it what I usually do :)

    Good Luck & Wish you a Nice Day!

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7 years ago, # |
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Just the thing I want in my semester break vacation!

Thanks A Lot!

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7 years ago, # |
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Recently I was learn Link Cut Tree. Have any list of problems set of link cut tree? Help Me to find out.

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7 years ago, # |
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is problem set arranged in order of ascending difficulty?

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    7 years ago, # ^ |
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    Good day to you,

    nope it is not. Some problems mighthave number next_to them, which is estimated difficulty, but it is just "a very wild guess" :).

    Wish you a nice day!

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7 years ago, # |
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Strongly Connected Components(SCC) http://mirror.codeforces.com/problemset/problem/427/C

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    7 years ago, # ^ |
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    Good day to you!

    Thank you very much. I've (hopefully) updated the topic!

    Wish you a nice day!

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7 years ago, # |
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In your MO part there is a problem(https://toph.ws/p/distinct-dishting). The link has changed. It's now https://toph.co/p/distinct-dishting. If you have listed any other problem from that site I think the link should be updated.

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7 years ago, # |
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Did you simply miss HLD or it's there but I can't find it?

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    7 years ago, # ^ |
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    Good day to you,

    HLD is there as part of "LCA"

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7 years ago, # |
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Can someone explain what is the solution for this problem? http://www.spoj.com/problems/AMR10J/

I am thinking a dp on tree solution, something like O(N*K^2) but that would be too slow. No clue how the problem tag you mentioned, of DAG, would help. Isn't the graph in the form of a forest, with each tree having only one cycle?

Hints would probably be more appreciated though, seems like an interesting problem. :)

EDIT:

Help no longer needed! Managed to solve it in just O(N) and N is only <= 100. Pretty neat. In case anyone is interested, here is the code: https://pastebin.com/GrvCjSwt

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    6 years ago, # ^ |
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    As I was browsing here this problem caught my eye. I have made not much progress in a week or so other than determining the shape of each component (multiple trees hanging from a cycle).

    Could you please explain your dp idea? For example, what does DP[i][j] represent?

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7 years ago, # |
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Is there a CHT list?

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    7 years ago, # ^ |
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    Good day to you,

    sorry but not :'( Maybe there is something in Number Theory but I wouln't bet on many problems

    Wish you a nice day!

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6 years ago, # |
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Some more easy problems on Josephus : http://www.spoj.com/problems/WTK/ (Easy) http://www.spoj.com/problems/DANGER/ (Easy) http://www.spoj.com/problems/POCRI/ (Medium) http://www.spoj.com/problems/NG0FRCTN/ (Medium)

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    6 years ago, # ^ |
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    Good day to you,

    thank you very much. I wanted to reply after update yet sadly I'm getting 504 Gateway Time-out when trying to update for more two week already so sorry, and I hope it will get better once :'(

    Wish you a nice day!

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      6 years ago, # ^ |
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      It's my pleasure to get a reply from you! Great efforts in Compilation, really useful.

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6 years ago, # |
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Bridge Problem : http://mirror.codeforces.com/contest/1000/problem/E

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6 years ago, # |
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the question ada and power mentioned in friedvald algorithm section is giving wa for test file 15. please give some insights

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6 years ago, # |
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I have an Idea for you if you can do it.

  • I love solving problems from CF so when I open any category I search for CF problems and start solving them.
  • So my idea is to divide the problems in each category by the problem's website to make it easier to navigate.

Example :

dfs
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6 years ago, # |
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What does "L-mex" stand for in the segment tree section?

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    6 years ago, # ^ |
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    Good day to you,

    not much sure now — yet imho firstly there is some check with "L" parameter... and then probably getting "MEX" (minimal unused value).

    dunno — sorry :P

    Wish you a nice day ^_^

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6 years ago, # |
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hey, Can somebody help me with this problem Ada and Zoo

thanks :)

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    6 years ago, # ^ |
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    Good day to you,

    I would recommend you to take peek onto Steiner's Tree -> That shall be a great start ^_^

    Good Luck

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6 years ago, # |
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You should erase most categories from that list, because they are useless. Problemsetters no longer know what most of those categories are.

RMQ? KMP? LCA? What's all that? Might be some names for prehistoric problem solving techniques. All you need today is math, some thinking and three lines of code.

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6 years ago, # |
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I wish there were tutorials and solutions gathered for these problems :D

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6 years ago, # |
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What do you mean by hard/weak children? (in category 'topo', problem : spoj — 'CODESPTI')

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    6 years ago, # ^ |
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    Good day to you,

    well it was just kind of hint that in my approach you should distinguish nodes by their "shape" (which recursively applies)

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6 years ago, # |
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This one link could be interesting for persistent segment tree, for me it was...

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5 years ago, # |
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Please anyone give the link for problem 4956 [LA] . category : friedvaldAlgorithm

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5 years ago, # |
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I wrote a script to save all these questions along with their categories in a CSV file using python. Here is Pastebin link if you would like to use it.

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    5 years ago, # ^ |
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    I have saved your code on one of the repls, so you can just run and get the file directly (Link).

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      5 years ago, # ^ |
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      Thanks a lot! I shall add this link to my blog post if you have no problem.

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I think 576C is the most classical problem for MO's algorithm. So that's good to add it.

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5 years ago, # |
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-Morass- Can You say the total numbers (digit) :p of the problem listed here :D :D

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5 years ago, # |
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sorry my bad

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Thank you so much! I will definitely use this to structure my training. I appreciate all of the div 1 codeforcers helping out the rest of us!

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Hey -Morass- I've been trying to solve SPOJ — JUSTAPAL from the palindrome category using hashing + binsearch and then using nlogn SA + LCP + RMQ to get first and second mismatches and do some swaps. However I'm still getting TLE. Is there something to take into account for getting a faster solution. Can you please share a hint or your code about this one? Thanks! :)

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    5 years ago, # ^ |
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    Hmm okkey not sure whether BS + Hashing wouldn't time-out actually..

    Anyways SA+LCP+RMQ seems to be right solution. Guess with this (you can check your local build-time, whether it is not too slow already --> it is NlogN so it shall be slowest part I guess, but shall not time-out: So try it/check it out), it shall be linear: For each possible "middle" check how far you can have palindrome -> then "repair" the next part and check whether the palindrome can continue .. and possibly whether it was "repaired" by swaping the next broken part and possibly check how far you get next....

    So in the end (omitting build part): There shall be (+/-) 2N such processes, with at most 3 RMQ questions and a few peeks to array: So this part shall be linear and hopefully shall not time-out.

    So if this is your approach, then it shall be fast enough (if there is no mistake :O )

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5 years ago, # |
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-Morass- could you plz include scanline algorithm problems like this https://www.codechef.com/problems/LAZER

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5 years ago, # |
← Rev. 2   Vote: I like it +11 Vote: I do not like it

I was searching for randomization problems, but couldn't find a randomization category here. :( After expanding the categories and searching for "random", I found 9 problems, but I think it deserves its own category.

There are some problems that have very nice randomized solutions, and sometimes they aren't the intended solutions, but I believe randomization can sometimes be very powerful and interesting.

If I remember some of them I'll add them here. Kindly if anyone has such problems, post them too.

I found the following blogs that have some randomization problems/tutorials:

On a separate note, I believe it'll nice if each topic has its list of tutorials too. Most have easily-reachable nice tutorials, but some do not, and some tutorials are much better than others, so I believe it'll be nice if there's like a list of high-quality tutorials per topic, along with short comments if needed.

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5 years ago, # |
  Vote: I like it -8 Vote: I do not like it

Can you please arrange them in groups? for example, graphs, geometry etc.

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5 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

FFT Questions :

https://mirror.codeforces.com/problemset/problem/1096/G

https://mirror.codeforces.com/problemset/problem/632/E

https://mirror.codeforces.com/problemset/problem/1251/F

I found these nice ..

You may add these too

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4 years ago, # |
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Commenting to save the blog! Thank you!

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    4 years ago, # ^ |
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    You can add it to favorites, just click the star at the end of the blog (near the contribution count).

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4 years ago, # |
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thanks a ton :)

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4 years ago, # |
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Thanks a lot for these! These are really useful for beginners like me.

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4 years ago, # |
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hi and thanks for amazing blog post -Morass-. can you explain your segment Tree solution for 915E please?

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4 years ago, # |
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whats is total number of problems here? Has anybody counted ?

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    4 years ago, # ^ |
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    2870

    Spoiler
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4 years ago, # |
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"Unable to parse markup [type=CF_MARKDOWN]"- Is this the actual post or is the link broken??

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    4 years ago, # ^ |
      Vote: I like it +21 Vote: I do not like it

    Hey ye: no idea what is wrong (not much verbose x( )

    Here is the source to the post -- so you might either refer to it, or if anybody sees any visible failure — you can ping me and I'll try to fix x)

    Have nice day ^_^

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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      The link gives me this error -_-

      Error, this is a private paste. If this is your private paste, please login to Pastebin first.

      Could you solve it, please?

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4 years ago, # |
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Thank you for your response got the pastebin link

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4 years ago, # |
← Rev. 2   Vote: I like it +14 Vote: I do not like it

The problems aren't enclosed properly in the spoiler boxes currently. -Morass-

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4 years ago, # |
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Isn't there a topic for Heavy-Light-Decomposition? I haven't found it

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3 years ago, # |
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I can't think of the solution to the problem "CODESPTI" in topic "topo". Could someone tell me how to do it?

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2 years ago, # |
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how to approach graph modeling problems?

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2 years ago, # |
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That's really great :)

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21 month(s) ago, # |
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Great problemset

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10 months ago, # |
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some problem links are not working....

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4 months ago, # |
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thanks

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2 months ago, # |
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d

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