3SUM

#include <iostream>
#include <map>
#include <unordered_map>
const int N = 5000; 
long long values[N];

int main(){
  int n;
  long long s;
  std::cin >> n >> s;
  for (int i = 0; i<n; ++i){
    std::cin >> values[i];
  }

  for (int i = 0; i<n; ++i){
    std::unordered_map<long long, int> exists;
    for (int j = 0; j<i; ++j){
      if (i==j) continue;
      long long rem = s-(values[i] + values[j]);
      auto it = exists.find(rem);
      
      if (it != exists.end()){
        std::cout << i+1 << " " << j + 1 << " " << it->second  + 1 << "\n";
        return 0;
      }
      exists[values[j]] = j;
    }
  }
  

  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: TIME LIMIT EXCEEDED

Test 14

Verdict: TIME LIMIT EXCEEDED

Test 15

Verdict: TIME LIMIT EXCEEDED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: TIME LIMIT EXCEEDED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: TIME LIMIT EXCEEDED

Test 22

Verdict: TIME LIMIT EXCEEDED

Test 23

Verdict: ACCEPTED