What is wrong with the code - the RE flag?

Question

C++ / C#

There is a task: the file "input.txt" is fed to the input. The first line of this file contains the number k, the second line contains a sequence of numbers written with a space, numbers in the range 0 < n < 99999999. If the sequence contains two numbers whose sum is equal to k, then write to the file "output.txt" 1, otherwise - 0.

#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>

using namespace std;


int main()
{
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  unsigned int k, num, ind=0, mxN=2e6;
  
  unsigned int arr[mxN];
 
  ifstream in("input.txt");
  ofstream out("output.txt");

  in >> k;

  while (in >> num)
  {
    arr[ind] = num;
    ++ind;
  }
  if (!k || !ind) return 0;
  // 2 pointers
  sort(arr, arr+ind);
  int l = 0;
  int r = ind - 1;
  while (l != r)
  {
    if (arr[l] + arr[r] < k) 
    {
      ++l;
    }
    else if (arr[l] + arr[r] > k)
    {
      --r;
    }
    else {
      out << "1";
      out.close();
      return 0;
    }
  }

  out << "0";
  out.close();
  return 0;
}

Wrote this solution. It is working, but on one of the tests it crashes with the RE flag. With what it can be connected?

Answer 1

unsigned int arr[mxN];
Shield? A huge array on the stack, and even a variable size? This is anything but code that conforms to the C++ standard. Vector must be used.
Upd: Here you need to use unordered_set. Count numbers into it, then walk through it and see if there is an opposite number.