3SUM

#include <iostream>
#include <unordered_map>
const int N = 5001; 
long long values[N];

struct pair{
  long long a;
  long long b;
  int i;
  int j;
};

pair pairs[N*N];

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

  std::unordered_map<long long, int> valsPairs;
  for (int i = 0; i<n; ++i){
    for (int j = 0; j<i; ++j){
      if (i==j) continue;
      pair p{values[i], values[j], i, j};
      pairs[i*N + j] = p;
      valsPairs[s-(p.a + p.b)] = i*N + j;
    }
  }
  


  for (int i = 0; i<n; ++i){
    auto it = valsPairs.find(values[i]);
    if (it != valsPairs.end()){
      int idx = it->second;
      pair& p = pairs[idx];
      if (i!=p.i && i!=p.j){
        std::cout << i+1 << " " << p.i + 1 << " " << p.j + 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: TIME LIMIT EXCEEDED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: TIME LIMIT EXCEEDED

Test 17

Verdict: TIME LIMIT EXCEEDED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: TIME LIMIT EXCEEDED

Test 21

Verdict: TIME LIMIT EXCEEDED

Test 22

Verdict: TIME LIMIT EXCEEDED

Test 23

Verdict: ACCEPTED