3SUM

#include <bits/stdc++.h>
using namespace std;
using ll = long long;

const int N = 5000;
unordered_set<int> S;
pair<int,int> T[N];

int main() {
  ll n, x;
  cin >> n >> x;
  for (int i = 0; i < n; ++i) {
    int k;
    cin >> k;
    T[i] = {k,i};
  }
  sort(T,T+n);
  int ind = 0;
  for (int i = 1; i < n; ++i) {
    if (T[i] != T[i-1]) {
      T[ind++] = T[i-1];
    }
  }
  T[ind++] = T[n-1];
  n = ind;
  for (int i = 0; i < n; ++i) {
    for (int j = i+1; j < n; ++j) {
      ll k = x - T[i].first - T[j].first;
      if (k < 0) break;
      if (S.count(int(k))) {
        int ind = 0;
        while (T[ind].first != k) ++ind;
        cout << T[i].second+1 << ' ' << T[j].second+1 << ' ' << T[ind].second+1 << '\n';
        return 0;
      }
    }
    S.insert(T[i].first);
  }
  cout << "IMPOSSIBLE\n";
}

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: ACCEPTED

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: TIME LIMIT EXCEEDED

Test 23

Verdict: ACCEPTED