3SUM

#include <iostream>
#include <unordered_map>
#include <ext/pb_ds/assoc_container.hpp>

const int N = 5000; 
long long values[N];

int main(){
  std::cin.tie(NULL);
  std::ios_base::sync_with_stdio(false);
  int n;
  long long s;
  std::cin >> n >> s;
  __gnu_pbds::gp_hash_table<long long, int> exists; // faster than std::unordered_map
  for (int i = 0; i<n; ++i){
    std::cin >> values[i];
    exists[values[i]] = i;
  }

  for (int i = 0; i<n; ++i){
    for (int j = i+1; j<n; ++j){
      long long rem = s-(values[i] + values[j]);

      if (exists.find(rem)!=exists.end() && exists[rem] != i && exists[rem] != j ){
        std::cout << i+1 << " " << j + 1 << " " << exists[rem]  + 1 << "\n";
        return 0;
      }
    }
  }


  std::cout << "IMPOSSIBLE\n";
  return 0;

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: TIME LIMIT EXCEEDED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED