3SUM

#include <bits/stdc++.h>
using namespace std;

vector<int> aa;
map<int, vector<pair<int, int>>> mm;
set<int> mms;

int main() {
	int n, x;
	cin >> n >> x;
	for (int i=0; i<n; i++) {
		int a;
		cin >> a;
		aa.push_back(a);
		for (int j=0; j<(int)aa.size(); j++) {
			if (j == i || aa[i] + aa[j] >= x) continue;
			mm[(aa[j]+aa[i])].push_back(make_pair(j, i));
			mms.insert(aa[j]+aa[i]);
		}
	}
	for (int i=0; i<(int)aa.size(); i++) {
		int rem = x - aa[i];
		if (!mms.count(rem)) continue;
		for (auto p : mm[rem]) {
			if (i != p.first && i != p.second) { 
				cout << p.first+1 << " " << p.second+1 << " " << i+1 << endl;
				return 0;
				
			}
		}
	}
	
	cout << "IMPOSSIBLE" << endl;
	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: TIME LIMIT EXCEEDED

Test 13

Verdict: TIME LIMIT EXCEEDED

Test 14

Verdict: TIME LIMIT EXCEEDED

Test 15

Verdict: TIME LIMIT EXCEEDED

Test 16

Verdict: TIME LIMIT EXCEEDED

Test 17

Verdict: TIME LIMIT EXCEEDED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: TIME LIMIT EXCEEDED

Test 21

Verdict: TIME LIMIT EXCEEDED

Test 22

Verdict: TIME LIMIT EXCEEDED

Test 23

Verdict: ACCEPTED