Only 8 testers? Interesting.

As a tester and an author of one problem, feecIe6418 orz.

As a tester, I can't wait to see PinkieRabbit's performance!

feecIe6418 orz.

waiting for a perfect contest!

As a tester, good luck!

As a human, ShuiLaoshi is a great bot!

good

I love global rounds so much ^_^

-

2:15? The only upside is that my suffering will end sooner.

:D

Hoping to become CM this round!

UPDATE: Messed up :(

It's known that global rounds used to have problems ranging from Div. 3 to Div. 1. This seems to be the first global round after Div. 4 hosted regularly. So will there be some Div. 4 problems in it?

Thanks.

Hoping to turn expert this round!

()

As someone having a discrete maths and linear algebra quiz, i am scared

looking forward to become Specialist this round! The expectations are very high.

cqoier Orz

Is this a Chinese Poisonous OI round again?

Ha! Dodged the collision with Project Euler Problem 804 without even conducting the round myself

Looking forward to some positive delta after falling to Pupil from Expert.

probably going to loose ratings again but ok

Can I finally reach CM? Lets find out.

pinkrabbit

I just hope that the problem statements tests our logic not English comprehension

Good luck!

Why there are always 4 out of 7 >1700 tasks in global rounds, maybe i am too stupid for that stuff...

Not complainig (still top 1000 solving A-E, not bad), just venting that python DESTROYED ME this round LOL, with amount of penalties that could kill a gorilla xD. Not sure how penalties can kill a gorilla, but damn it I believe they could.

Oh and forgot to say — GREAT ROUND!!! LOVED the problems. All of them were cool. Very cool.

How I solved E:

1. Realized the number of dolls that need to be moved from a white cell is just the number of paths to that cell from $$$(0,0)$$$.
2. Coded the simple $$$O(\sum{a_i})$$$ idea to verify I'm not missing something.
3. "Oh great Wolfram Alpha, please tell me how to simplify this equation!"

How does an educational problem like this appear in a rated contest, let alone a global round...

E = mathforces.

How to solve F?

Why did you have to make such strict constraints in D? Why 5e5? 1e5 should be enough to prevent N^2 solutions, maybe 2e5

Couldn't fit asymptotically correct but constant-suboptimal solution in python and had to rewrite it in c++.

UPD: nevermind, my solution was not asymptotically correct

problem A was so similar to leetcode biweekly 81 q3 which occurred at the same time its insane

Couldn't submit D T_T btw loved A-D, even E seemed noice Great round ⊂(◉‿◉)つ

Why does Jiangly submit A, B and C almost at the same time? Gives the others a head start?

I bet most of the submissions for problem B would have received WA on pretest 2 (includig me..XD)! They had us in the first half!

Nice questions! A and B were not straight, yet they were easy I couldnt do C tho...and I was about to finish D but looks like getting the minimum was problematic lol, maximum was working fine (time complexities considered)... :((( Could have been a long jump I guess xD But looks like i will take a minus now

update: I got a + lol

D has weak pretests. In the worst case, my first submission takes $$$O(N)$$$ time to find the next vertex, so its overall complexity is $$$O(N^2)$$$, but it passed pretests.

very much enjoyed orz :)

this is my first time in the contest.although i ac one ，i am also very happy.

So, here is some feedback on the problems:

• A: Ok easy problem.
• B: Ok easy problem.
• C: Nice and clean. Somewhat educational, but still enjoyable.
• D: Cool problem, with the appropriate difficulty. I saw rather quickly the main observation (it is convenient to go greedy) and then I spent an eternity implementing it.
• E: Uhm, rather standard. First thought after reading: this must be related to the number of path from $$$(0, 0)$$$ to something. Then, after ~15 minutes I could pinpoint exactly to what paths it is related to. Felt a bit easy as an E (but actually also B, C, D felt a bit easy, so it fits).
• F: First, mandatory attachment: What a run! I hope I pass systests, since it would be the closest-call (10 seconds) I have ever experienced on cf. Wonderful problem, it was hard for me and I am proud of myself for solving it. I spent quite some time thinking and realized only after more than 40 minutes how to detect a leaf. Then I (barely) managed to implement it before the end of the contest.

Overall, the contest was well-prepared and the statements were clear. Even though I am not a fan of the first 5 problems, which are somewhat standard, I had so many emotions solving F that I liked the whole contest a lot! Thanks to the authors.

why there were n+1 integers in input instead of n in E?(ಥ﹏ಥ) i missed an AC due to that (ಥ﹏ಥ)(ಥ﹏ಥ)(ಥ﹏ಥ)

Feel so good to solve E, at least pass the pretests! After running nowhere with some vague idea of formal power series or polynomials, I started to notice the pattern of Binomial coefficients when drawing on paper some examples, and then search around for binomial identities and finally find a match! Never feel so certain that I still have potential to improve my mathematics skills~

Hope no FST~

E was easier to think after linking to pascal's triangle

Stumbling upon the orz problem was so cute! (even though it was trivial)

Made the factorial array of 2*10^5 instead of 4*10^5(a[i]+i). Missed E! :'(

"Find the maximum possible value of the maximum value in a after any number (possibly zero) of operations."

In Problem A this line mislead me into thinking that we have to maximize the maximum value of the original array, so I took the maximum value say "V" in the array and expected V|Z to be the answer, but after spending like an hour and half I realized that the answer would be the maximum attainable value of any value in the array, I would like to request the authors to write clearer statements to avoid such confusion, like the line mentioned above could be replaced by, "Find the maximum possible value in a after any number (possibly zero) of operations." Those who faced the same difficulty can upvote the post so that it reaches the authors. Peace.

• Weird statements
• swap(D,E)
• Weak pretests for D
• E is classical
• F is boring and implementation is annoying
• G is classical(get the duel problem and solve it) and implementation is much more annoying

Hyper Chinese round! Very enjoyable!

Does E have a solution if there is no non-increasing $$$a_i$$$ constraint?

I tried to solve D with pie for pie (bfs trick) + sparse table but it gets TLE on test 2. Can somebody explain why? Thanks in advance. Submission link:

Ratings updated preliminarily. We will remove cheaters and update the ratings again soon!

I wonder why this round is chosen to be GR. There are definitely better choices like Round 796.

a great contest :) thanks

Fast tutorial and excellent problems today. Loved the contest.

why my rating get decreases from 777 to 736 though I have solved the 'A'.

Unofficial T-shirt winners list:

(Used the same method as: https://codeforces.net/blog/entry/102081?#comment-908978)

I was double-checked, but I submitted the code 16 minutes earlier than the other person. Code 161763398,161770959. I don't know anyone else, but I suspect that someone found my code in room and sent it to someone else. Hope for a retrial.

In re-sentencing, please restore my ranking first, rather than according to a question did not pass the ranking calculation. I could have gained 75 points, but now I've lost 175.

Please tell why my solution is giving TLE. Shouldn't N((log(N))^2) pass? My solution Problem

Why didn't you restore my rank first when you re-evaluated my score? Your unreasonable behavior caused me a lot of trouble. I hope you can help me solve it.

Congratulations to tshirts winners! In a few weeks you will be contacted via private messages with instructions to receive your prize.

As usual, we used the following two scripts for generating random winners, seed is the score of the winner.

#!/usr/bin/python3

import requests

contests = [1696]

# [1, rank_start) get tshirts with no random
# [rank_start, rand_end] get random tshirts
rank_start = 31
rank_end = 500

n_random_tshirts = 20

def is_eligible(contest, party_name, rank, points, problems, last_submission_time):
    return (rank <= rank_end)

def get_party_name(party):
    if 'teamName' in party:
        return party['teamName']
    else:
        return party['members'][0]['handle']

class Party:
    def __init__(self, contest, name, rank, points, last_submission_time):
        self.contest = contest
        self.name = name
        self.rank = rank
        self.points = points
        self.last_submission_time = last_submission_time
    
    def __lt__(self, other):
        if self.contest != other.contest:
            return self.contest < other.contest
        if self.rank != other.rank:
            return self.rank < other.rank
        if self.last_submission_time != other.last_submission_time:
            return self.last_submission_time < other.last_submission_time
        if self.name != other.name:
            return self.name < other.name
        return False

eligible = []

for contest in contests:
    rows_to_fetch = 20000
    r = requests.get('https://codeforces.net/api/contest.standings?contestId=%d&from=1&count=%d&showUnofficial=false' % (contest, rows_to_fetch))
    if r.status_code != 200 or r.json()['status'] != 'OK':
        print('Error: failed to fetch results of contest %d.' % contest)
        exit()
    results = r.json()['result']['rows']
    if len(results) == rows_to_fetch:
        print('Error: results are too long in contest %d, increase limit.' % contest)
        exit()
    for row in results:
        party_name = get_party_name(row['party'])
        rank = row['rank']
        points = row['points']
        problems = 0
        last_submission_time = 0
        for problem in row['problemResults']:
            if problem['points'] > 0:
                problems += 1
                last_submission_time = max(last_submission_time, problem['bestSubmissionTimeSeconds'])
        if is_eligible(contest, party_name, rank, points, problems, last_submission_time):
            eligible.append(Party(contest, party_name, rank, points, last_submission_time))

eligible = sorted(eligible)

for i in range(1, len(eligible)):
    if not eligible[i - 1] < eligible[i]:
        print('Error: unable to sort eligible at positions {} {}.'.format(i, i, i + 1))
        exit()

#print('Eligible for tshirts are:')
#for idx, party in enumerate(eligible):
    #print('{} {} {} {} {}'.format(idx + 1, party.contest, party.rank, party.name, party.last_submission_time))


seed = eligible[0].points

print('Total %d eligible, seed is %d.' % (len(eligible), seed))

import subprocess
testlib_rand = subprocess.run(['./randgen', str(seed), str(len(eligible)), str(rank_start - 1),    str(n_random_tshirts)], stdout=subprocess.PIPE)
winners = list(map(int, testlib_rand.stdout.split()))

print()
print('Winners are:')
for place in winners:
    print('{}'.format(eligible[place - 1].name))

#include "testlib.h"
#include <iostream>

using namespace std;

int main(int argc, char *argv[])
{
    if (argc < 4)
    {
        cout << "Command line arguments are: seed length prefix nwinners" << endl;
        return 1;
    }
    int seed = atoi(argv[1]);
    int len = atoi(argv[2]);
    int prefix = atoi(argv[3]);
    int nwinners = atoi(argv[4]);
    rnd.setSeed(seed);
    
    set<int> winners;    
    while ((int)winners.size() < nwinners)
        winners.insert(rnd.next(prefix + 1, len));

    for (int i = 1; i <= prefix; ++i)
        winners.insert(i);
    
    for (auto winner: winners)
        cout << winner << " ";
    cout << endl;
    
    return 0;
}