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Tutorial of Educational Codeforces Round 19

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Codeforces (c) Copyright 2010-2021 Mike Mirzayanov

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Can't understand C's explanation.

True.. it should have been like: Extract the first character of s and append this character with t. Extract the last character of t and append this character with u.

rather then://this don't make any sense to me, append t/u with character? t and u are empty! Extract the first character of s and append t with this character. Extract the last character of t and append u with this character.

Alternately F can be solved with Divide and Conquer Optimisation as well.

I seem to get TLE with Divide and Conquer Optimization , was any pruning required to pass the time limit ?

It's Complexity O(n^2logn)?

Problem E can also be solved offline with an O(n) memory complexity, by sorting the queries w.r.t k and solving all queries with the same k together using dynamic programming.

Intuitions about the time complexity:I don't know exactly how to calculate a generalized upper bound for the time complexity, but I had the intuition that it would fit in time using this approach

Implementation:26497630(1/2 + 1/3 + 1/4 + ... + 1/n) = O(logn)

Oh yes you're right, it's logn. I got confused about it. Thanks.

dropped

Note, it's order.

Hint for proof:

I didn't learn integrals in school yet. But I could see that your formula is not correct:

Seems that functions are crossing and lying nearby but not equal.

click

Actually, it's not . And you can see that the difference between (a.k.a harmonic number) and go smaller as n go bigger. It will eventually equal 0 when n big enough. Furthermore, is smaller than and as we can ignore constant, we can say it's O(logn)

Thank you, I thought

`log`

always means`log2`

in informatics, not`ln`

.Here is another proof: 1/1+1/2+1/3+1/4+1/5+1/6+1/7+..<1/1+1/2+1/2+1/4+1/4+1/4<1+1+1...+1=log n and 1/1+1/2+1/3+1/4+1/5+1/6+1/7+1/8. <1+1/2+1/4+1/4+1/8+1/8+1/8+1/8<1+1/2+1/2...+1/2=log n/2 So 1/1+1/2+...+1/n is O(log n)

But the time is worse than using sqrt precalculation..

Look, if all k will be different your code will make this Q times:

`memset(dp, -1, sizeof(dp));`

It is optimized of course but making your complexity bigger.Sorry for necropost, but in this case your solution works in

`O(n sqrt(n))`

time:Because your solution takes

O(n) operations per one block, but this isO(n) memory, u are righthi excuse me but if all queries have different k's then you say if (k != prev) {memset(dp, -1, sizeof(dp));} so you basically fill the dp with -1's in every query but isn't this O(n^2)? could you explain why you don't get TLE? thanks ,and sorry to bothering you :D

can anyone please explain Problem c editorial

I understand that this contest is quite old but 797C - Minimal string has a typo and says "lexigraphically" instead of "lexicographically". I'm not sure if this is correct way to report typos but don't see any other.

Fixed, thanks.

Help me please, I don't know what is wrong with my code I am getting the wrong answer for test-case 17 of the question Minimal String (question C), here is my solution link. https://ideone.com/mc0zjt

can you give me some test cases I am getting WA at test 21?

In Problem C's solution, instead of — 'any letter left in string s' in

"Pop letters from the top of stack and push them to answer while they are less or equal than any letter left in string s."

It should be --- 'while they are less or equal than all the letters left in string s'.

Can anyone give me a DP solution for Problem B? Why is this approach wrong?

Here is the DP solution in case you need it still.

http://codeforces.com/contest/797/submission/41646680

Someone help me please getting the wrong answer. what is wrong with my code? here is my ideone link to my code https://ideone.com/mc0zjt

Can anyone tell me where am I wrong in this code. It is giving wrong answer for an input. I am not getting why my code is giving wrong output for that input. Printing o before n. http://codeforces.com/contest/797/submission/33380771

Why i am getting WA on test Case 16 in problem no C? Here is my submission 35083720.Anyone please help.

can you give me some test cases I am getting WA at test 21?

can anyone can give me some test cases on the question C I am getting WA at test 21?

test this maybe it would help :D

input: czzbaz output should be: abzzcz

Wow dude, thanks for that test case. I was forgetting like half the problem. Thanks.

no bro it didn't work for me

Yeah mate, really helpful test case. After testing with this one got AC

yes same i have given 2 days to the ques

Why am I getting a runtime error when I run the following code ?

http://codeforces.com/contest/797/submission/50471261

Someone please explain problem 797F . Can't understand the editorial!!!

someone please simulate this case:

acdb

s = acbd, t = '', u =''

s = cbd, t = a, u = ''

s = bd, t = ca, u = '' [first char 'c' was taken and string t was appended]

s = b, t = dca, u = ''

if the last character is to be extracted from t then it is 'a' and it will be in the last position of u which is not the output given in the test case.

i think i am misinterpreting the following lines

"Extract the first character of s and append t with this character.

Extract the last character of t and append u with this character."

Problem statement of C is very misleading. At first I could visualize string T nothing more than a queue and sorting seemed impossible.

problem cat first i had difficulty understanding the question and then with whatever i understood ,i wrote adp solutionbut it gotWA on TC 17.if anyone has dp solution or can find the error in my code , it will be helpful.96818512 is my submission.try this test case: eeddcbeeec

correct ans is: bcceeeddee while your code gives: bceeecddee

HI, can anyone help me to provide the time complexity of my solution to problem E,

Here is the link to my submission:- https://codeforces.com/contest/797/submission/100305884

I guess it is n√n,

I have solved for every k individually and for a particular k, i have solved for all different values of p and storing the result for traversed indices for every p.

I think that worst case complexity would be, n + 2*(n/2) + 3* (n/3) + ... x terms s.t. x*(x+1)/2 = q; Because as k increase from 1 to n I can solve for particular k and pparticular p in n/k time, so, if all k are different then it will be, n + n/2+n/3... = nlog(n), and if there are multiple p for same k then it will be (dup, no.of different p for same k), (n/k * min(dup,k)).