3SUM

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

using namespace std;

using pii = pair<int, int>;
using ll = long long;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    ll x;
    cin >> n >> x;

    vector<pii> a(n);
    for (int i = 0; i < n; ++i) {
      int aux; cin >> aux;
      a[i] = {aux, i+1};
    }

    sort(a.begin(), a.end());

    for (int i = 0; i < n - 2; ++i) {
        ll target = x - a[i].first;
        int left = i + 1;
        int right = n - 1;

        while (left < right) {
            ll current_sum = a[left].first + a[right].first;

            if (current_sum == target) {
                cout << a[i].second << " " << a[left].second << " " << a[right].second << endl;
                return 0;
            }
            if (current_sum < target) left++;
            else right--;
        }
    }

    cout << "IMPOSSIBLE" << endl;

    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