3SUM

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

const int N = 5e3 + 5;

int n, x;
int a[N];

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> x;
  vector<pair<int, int> > v;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    v.emplace_back(a[i], i);
  }
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      int y = x - a[i] - a[j];
      auto k = lower_bound(v.begin(), v.end(), make_pair(y, -1));
      if (k == v.end()) {
        continue;
      }
      if (k->first == y) {
        if (k->second != i && k->second != j) {
          cout << i << ' ' << j << ' ' << k->second << '\n';
          return 0;
        }
      } 
      if (k != v.end()) {
        k++;
      }
      if (k->first == y) {
        if (k->second != i && k->second != j) {
          cout << i << ' ' << j << ' ' << k->second << '\n';
          return 0;
        }
      } 
      if (k != v.end()) {
        k++;
      }
      if (k->first == y) {
        if (k->second != i && k->second != j) {
          cout << i << ' ' << j << ' ' << k->second << '\n';
          return 0;
        }
      } 
    }
  }
  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: 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: ACCEPTED

Test 23

Verdict: ACCEPTED