3SUM

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

using namespace std;

const int N = 5e3 + 5;

int n, x;
int a[N];
pair<int, int> p[N * N];

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  for (int i = 0; i < N * N; i++) {
    p[i] = make_pair(-1, -1);
  }
  cin >> n >> x;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  vector<int> v;
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      v.emplace_back(a[i] + a[j]);
    }
  }
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
  for (int i = 1; i <= n; i++) {
    if (binary_search(v.begin(), v.end(), x - a[i])) {
      int j = lower_bound(v.begin(), v.end(), x - a[i]) - v.begin();
      if (p[j] != make_pair(-1, -1)) {
        cout << p[j].first << ' ' << p[j].second << ' ' << i << '\n';
        return 0;
      }
    }
    for (int j = 1; j < i; j++) {
      p[lower_bound(v.begin(), v.end(), a[j] + a[i]) - v.begin()] = make_pair(j, i);
    }
  }
  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: TIME LIMIT EXCEEDED

Test 14

Verdict: TIME LIMIT EXCEEDED

Test 15

Verdict: TIME LIMIT EXCEEDED

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