3SUM

#include <bits/stdc++.h>

using namespace std;

int main() {

    long long n, x;
    cin >> n >> x;

    vector<long long> values(n);
    for (long long i = 0; i < n; i++)
        cin >> values[i];

    if (n < 3) {
        cout << "IMPOSSIBLE" << endl;
        return 0;
    }
    
    vector<long long> sorted_values(n);
    sort(sorted_values.begin(), sorted_values.end());

    for (long long i = 0; i < n; i++) {
        for (long long j = i+1; j < n; j++) {

            long long sum2 = values[i] + values[j];
            long long target = x - sum2;

            auto p = equal_range(sorted_values.begin(), sorted_values.end(), target);

            if (p.first == p.second)
                continue;
            
            long long k = *p.first;
            
            if (k != i && k != j) {
                cout << i+1 << " " << j+1 << " " << k+1 << endl;
                return 0;
            }

            else if (k+1 < n && sorted_values[k+1] == sorted_values[k]) {

                if (k+1 != i && k+1 != j) {
                    cout << i+1 << " " << j+1 << " " << k+2 << endl;
                    return 0;
                }

                else if (k+2 < n && sorted_values[k+2] == sorted_values[k]) {
                    cout << i+1 << " " << j+1 << " " << k+3 << endl;
                    return 0;
                }

            }

        }
    }


    cout << "IMPOSSIBLE" << endl;
    return 0;

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: ACCEPTED

Test 5

Verdict: WRONG ANSWER

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: ACCEPTED

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: WRONG ANSWER

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: WRONG ANSWER

Test 17

Verdict: WRONG ANSWER

Test 18

Verdict: WRONG ANSWER

Test 19

Verdict: ACCEPTED

Test 20

Verdict: WRONG ANSWER

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED