3SUM

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <climits>

typedef long long ll;

using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    // freopen("input.txt", "r", stdin); // TODO: REMOVE THIS YOU STUPID ****

    int n;
    ll x;
    cin >> n >> x;
    vector<pair<ll, int> > a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i].first;
        a[i].second = i + 1;
    }

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

    for (int i = 0; i < n; i++) {
        int l = i + 1, r = n - 1;
        while (l < r) {
            ll s = a[i].first + a[l].first + a[r].first;
            if (s == x) {
                cout << a[i].second << " " << a[l].second << " " << a[r].second << "\n";
                return 0;
            } else if (s < x) {
                ++l;
            } else {
                --r;
            }
        }
    }

    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