I had a question : is 6 minute time limit when downloading the input the only TL constraint on our programs ? Or is our source code ran with a tighter TL in the background after contest finishes ?

Very Excited to participate as it will be my very first contest. I request every one who is reading this give your time to each question and try to solve at least anyone of them, you have 3 days.

How many official submissions can we make ? I am asking this because what if we aren't able to submit the solution and output in 6 minutes due to any reasons like Power failure or network issues. Will we be able to submit the solution and output for some other input file in 6 minutes ?

Have a small question. Why dont we have the feature of submitting the code directly and run in your server rather than we running locally like almost all the sites (eg. Google Codejam) ?

can you tell where is the option to directly download raw input file without downloading the zip file, since I downloaded the zip file but was not able to extract the input file while I was trying to unzip it. It showed that an error occurred. Resulting, that I wasn't able to submit correct output file. Although I am pretty much sure that my solution is correct.

Can anyone tell me how can we run our code on an input text file and produce an output text file. I am using code blocks IDE on windows system. I also tried to manually create the output file by copy pasting input and output . But it's showing me presentation error.

it would be really cool to provide participation/progression certificates to participants like the CodeJam, ignore this if already implemented this year <3

what is the password?

imagine having a supercomputer and being able to run any kind of bruteforce...

I can't open the zip file by macOS default application.

After reading "Xs and Os" I wonder: why are we being encouraged in such a stupid cheating? :)

LoneFox when I am clicking on download validation input file, it starts downloading an index.html file. I have tried it 4 times. what should I do?

I had a doubt, when it says solution validated does it mean it was correct for that test case or it just means it was submitted successfully?

Solved!

Well, it's nice to see the "password protected input" feature implemented, but do you really need to use some fancy compression method that standard tools don't support? This is not nice:

$ unzip consistency_chapter_1_input.zip 
Archive:  consistency_chapter_1_input.zip
   skipping: consistency_chapter_1_input.txt  unsupported compression method 99

I've just registered on Facebook, but it buns me without any reason. I send them my photo and now have a message that they will check it , but I can't write a qualification round without an active Facebook account. Can they unblock my account faster or can I solve problems from this round without a Facebook account?

I would have preferred the source code to be submitted during validation to have a less panicky 6 minutes.

I created a Facebook account yesterday to participate in Hacker Cup. But Facebook has disabled it twice. When it disabled it the first time, it asked me to verify my phone number which I did. Now it is again disabled and Facebook asked for a photo which I have sent but I haven't received a reply. Now I am scared if my account remains disabled, I won't be able to participate. I have tried to create another account with different emails but all are disabled after they are created. Someone, please suggest what I should do. LoneFox please tell me something.

It might be a dumb question but do we have to solve any one of A1, A2,B, C1,C2 or one question is A1,A2,B and other is C1,C2.

After the end of the contest , how much time it will take to evaluate the submitted solutios?

What if I submit wrong code but correct output?(I ask for people to try to escape plagiarism checker(if it exists))

Why do you have to make the process more tedious by adding password protected inputs? I had issues with extracting the file.

Hi, I'd like to suggest you could add functionality to the scoreboard to filter by country, like Google Code Jam has.

Any intuition as to why this is wrong for C2? Only fails on large cases from the official test set.

Fix first path from $$$1$$$ -> $$$a$$$, last from $$$b$$$ -> $$$1$$$ for all possible distinct pairs of $$$a$$$, $$$b$$$ such that the paths don't have common edges, mark used edges and set $$$c_{x} = 0$$$ for nodes visited. Then $$$k - 1$$$ times, choose the next best path as the diameter with respect to $$$c_{x}$$$ using only unused edges (also mark used edges and set $$$c_{x} = 0$$$ for used nodes). This logically seems correct on examination with exchange arguments on $$$i$$$-th and $$$j$$$-th paths taken in optimal order, considering cases where paths have / don't have a common node separately.

Created video solutions for Facebook hackerCup solutions, if interested please checkout:

https://www.youtube.com/watch?v=hjTAshY60Xo&ab_channel=AlgoShalgo

Question:

How do I know if I qualified for Round 1 of the hacker cup? Would there be any official communication from the Facebook team?

I have a doubt that why they use 6 minute timer?

I appreciate the step taken be facebook to provide certificates but I would like to point out that the rank cannot be seen clearly/visible.

I don't have any idea. How did I miss this year hacker cup :(

I think that the problem B had weak testcases. I initially implemented a very suspicious messy solution, which passed the validation input:

import std.algorithm, std.array, std.container, std.conv, std.format, std.math, std.numeric, std.random, std.range, std.stdio, std.string, std.typecons, core.bitop;

void main()
{
  int t;
  readf!" %d "(t);
  foreach (casenum ; 1 .. t + 1) {
    int n;
    readln.formattedRead!" %d "(n);
    string[] grid;
    foreach (i ; 0 .. n)
      grid ~= readln.strip;
    int ans;
    int best = int.max;
    int best_count = 0;
    int[int] tmp;
    foreach (i ; 0 .. n) {
      bool good_h = true;
      bool good_v = true;
      int dots_h = 0, dots_v = 0;
      int vdot_pos;
      foreach (j ; 0 .. n) {
        if (grid[i][j] == 'O')
          good_h = false;
        else if (grid[i][j] == '.')
          dots_h++;
        if (grid[j][i] == 'O')
          good_v = false;
        else if (grid[j][i] == '.') {
          dots_v++;
          vdot_pos = j;
        }
      }
      if (good_v) {
        if (dots_v < best) {
          best = dots_v;
          best_count = 1;
        } else if (dots_v == best)
          best_count++;
      }
      if (good_h) {
        if (dots_h < best) {
          best = dots_h;
          best_count = 1;
        } else if (dots_h == best)
          best_count++;
      }
      if (good_h && dots_h == 1)
        tmp[i]++;
      if (good_v && dots_v == 1)
        tmp[vdot_pos]++;
    }
    foreach (k, v ; tmp)
      if (v > 1)
        best_count -= v - 1;
    if (best == int.max)
      writefln("Case #%d: Impossible", casenum);
    else
      writefln("Case #%d: %d %d", casenum, best, best_count);
  }
}

#include <iostream>
#include <map>
#include <vector>
#include <climits>
using namespace std;

int main()
{
  int t;
  cin >> t;
  for (int casenum = 1; casenum <= t; casenum++) {
    int n;
    cin >> n;
    vector<string> grid;
    for (int i = 0; i < n; i++) {
      string s;
      cin >> s;
      grid.push_back(s);
    }
    int best = INT_MAX;
    int best_count = 0;
    map<int, int> tmp;
    for (int i = 0; i < n; i++) {
      bool good_h = true;
      bool good_v = true;
      int dots_h = 0, dots_v = 0;
      int vdot_pos;
      for (int j = 0; j < n; j++) {
        if (grid[i][j] == 'O')
          good_h = false;
        else if (grid[i][j] == '.')
          dots_h++;
        if (grid[j][i] == 'O')
          good_v = false;
        else if (grid[j][i] == '.') {
          dots_v++;
          vdot_pos = j;
        }
      }
      if (good_v) {
        if (dots_v < best) {
          best = dots_v;
          best_count = 1;
        } else if (dots_v == best)
          best_count++;
      }
      if (good_h) {
        if (dots_h < best) {
          best = dots_h;
          best_count = 1;
        } else if (dots_h == best)
          best_count++;
      }
      if (good_h && dots_h == 1)
        tmp[i]++;
      if (good_v && dots_v == 1)
        tmp[vdot_pos]++;
    }
    for (auto const& [k, v] : tmp) {
      if (v > 1)
        best_count -= v - 1;
    }
    if (best == INT_MAX)
      printf("Case #%d: Impossible\n", casenum);
    else
      printf("Case #%d: %d %d\n", casenum, best, best_count);
  }
}

I wasn't happy about it and reimplemented my final solution in a different way before submitting. Turns out that my initial implementation was indeed buggy and failed the following simple testcase (printed "Case #2: 1 2" instead of "Case #2: 1 3"):

4
4
..XO
XXOO
XXXO
XXXO
3
.XX
OXX
...
3
XX.
OXX
..X
3
XX.
OOX
..X

But the official "xs_and_os_input.txt" input file from the contest was unable to catch this bug, so both buggy and correct implementations produced the same output. Did all participants receive the same "xs_and_os_input.txt" file or are the input files randomized to some extent?

Edit: Added a C++ variant of my broken solution if anyone wants to hack it (this code is very broken and should never pass any reasonable tests). But it happens to generate correct output for "xs_and_os_input_practice.txt" too (downloadable in practice mode now). Tagging LoneFox and SecondThread if they are maybe interested in fixing at least the practice mode.

How can I see the leaderboard for a specific country?

Could someone (un)justify this idea for C2? There are N * (N - 1) / 2 paths in a tree. Consider all the paths as vertices in a new graph, where weight of a vertex is sum of the corresponding path. Now, two vertices in a new graph will be connected if their corresponding paths share at least one edge. Then the problem reduces to finding maximum weight independent set in the new graph (with a restriction that at least one path must contain vertex 1 in original graph and cardinality of the set must be at most K). But the new graph is bipartite, so one can find maximum weight independent set by means of maximum matching which can be done efficiently in bipartite graphs by some max flow I believe. The thing is, I don't know if it's possible to use maximum matching for finding maximum weight independent set (it seems possible for unweighted case), i. e. by somehow tweaking the max flow algorithm. In addition, it could be hard to ensure that path with a root in original graph would be a part of the max independent set. But is this solution path correct in general?

The rank in my certificate isn't visible clearly. Can you fix this?

I solved C1 using information from direct children of node 1 (counting edge cases of having single child seperately). I have not done much question on tree-dp. Can anyone explain simpler method for C2.

what is the criteria of getting t-shirt LoneFox?

Surprisingly, my C2 has passed, while C1 has failed with the same solution. I suspect weak tests for C2.

For C2 I used O(N*K*K) DFS-based DP, where for every node V, DP[V][i] is the max sum taken from the corresponding subtree using at most i paths (with disjoint sets of edges), where i is either an integer or an integer with a 'half' (the latter means 'open' path extended outside the subtree). The last step in the solution is correct interpretation of DP[1] for obtaining the result (we either add 1 or not depending on whether a found set of paths passes through the vertex 1).

Has anyone experience the same or does know why my C2 solution doesn't work for C1?

How do I know if I have advanced to Round 1? When I open the Round 1 page, it shows "You have not registered for this contest". I was able to solve 1 question & it is correct.

Is it only me who solved C1 with LCA? LCA seems unnecessary here.

Hi, I'm trying to understand a C2 solution. I can't get the concept of extension points for the paths in the subtree. I understand that the path, which has one endpoint (extension point) in the subtree root, can be extended from the parent, by using parent-child edge. However, I noticed that there is a dp state, which has 2 such extension points — could somebody explain this situation? How could the path in the subtree be extended in 2 places?

I have a doubt regarding the round 1. Does the 6 minute timer appear only once and after that time, we cannot submit for that problem? I tried running my A1 solution and I use online codechef IDE and the input was too large for that and the timer ended before I could install a local compiler on my laptop and I can't submit for that problem again :(

Does after solving A1 and A2 , is it guaranteed that I am qualified for round 2.

How to run the input files for a1 every compiler says input too large.LoneFox

I was confident about the code which I wrote for A2 , it passed validation tests but while compiling the code crashed on sublime Text . What precaution should I take such that it does not crash for remaining problems ?

I got a problem in the final input file of the problem A1("Weak Typing — Chapter 1") of round 1 . The characters of the file are different from the input format of the problem.

Is this a problem with my PC ?

LoneFox Pls Help !!