3SUM

#include <bits/stdc++.h>

using namespace std;


int main() {
	int n;
    long long x;

	cin >> n >> x;

	vector<pair<long long, int>> arr;

	for (int i = 1; i <= n; i++) {

		long long a;

		cin >> a;

		pair<int, int> p;

		p.first = a;

		p.second = i;

		arr.push_back(p);

	}

	sort(begin(arr), end(arr));

	for (int i = 0; i < n; i++) {

		int l, r;

		l = 0;

		r = n - 1;

		while (l != r) {

			int target;

			target = x - arr.at(i).first;

			if (l != i && r != i && arr.at(l).first + arr.at(r).first == target) {

				cout << arr.at(i).second << " " << arr.at(l).second << " "

				     << arr.at(r).second << endl;

				return 0;

			}

			if (arr.at(l).first + arr.at(r).first < target) {

				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