3SUM

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;

int main() {
	int n, x;
	cin >> n >> x;
	vector<pair<int, int>> v;
	for (int i = 1; i <= n; i++) {
		int a;
		cin >> a;
		pair<int, int> p = make_pair(a,i);
		v.push_back(p);
	}
	sort(begin(v), end(v));
	for (int i = 0; i < n; i++) {
		int l = 0;
		int r = n - 1;
		while (l != r) {
			int obj = x - v.at(i).first;
			if (l != i and r != i and v.at(l).first + 
                v.at(r).first == obj) {
				cout << v.at(i).second << " " << v.at(l).second << " "<< v.at(r).second << endl;
				return 0;
			}
			if (v.at(l).first + v.at(r).first < obj) {
				l++;
			} else {
				r--;
			}
		}
	}
	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