3SUM

#include <bits/stdc++.h>

using namespace std;
#define ll long long


int main() {

    ll n, x;
    cin >> n >> x;

    vector<ll> a(n);
    for (auto &v : a) cin >> v;

    map<ll, vector<int>> m;
    for (int i = 0; i < n; i++) {
        m[a[i]].push_back(i);
    }

    for (auto &[k, v] : m) {
        reverse(v.begin(), v.end());
    }

    bool possible = false;
    for (int i = 0; i < n; i++) {
        m[a[i]].pop_back();
        for (int j = 0; j < i; j++) {
            int g = x-a[i]-a[j];
            // cout << i << " " << j << " " << x - a[i] - a[j] << " " << endl;
            if (m.count(g) > 0 && m[g].size() > 0) {
                cout << j+1 << " " << i+1 << " " << m[g][0]+1 << endl;
                possible = true;
                break;
            }
        }
        if (possible) break;
    }

    if (!possible) {
        cout << "IMPOSSIBLE" << endl;
    }


}

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