3SUM

#include <bits/stdc++.h>

using namespace std;

//Definitions for quicker writing
#define REP(i,a,b) for (int i = a; i < b; i++)
#define PB push_back
#define MP make_pair
#define F first
#define S second

//Typedefs for quicker writing
typedef long long ll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef pair<int,int> pi;
typedef pair<long long, long long> pl;

//Max values
const long long lmx = LLONG_MAX;
const int imx = INT_MAX;

int main() {
	//IO optimization
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	//Input definition
	int n;
	ll sum;

	//Read In
	cin >> n >> sum;

	ll val[n];
	REP(i,0,n) cin >> val[i];

	//Main part
	bool imp = true;
	ll first, second, third;
	string res = "";
	REP(i,0,n-2){
		if(!imp) break;
		unordered_set<ll> s;
		map<ll,ll> ref;
		ll cursum = sum - val[i];
		REP(j,i+1,n){
			ll need = cursum - val[j];
			if(s.find(need)!= s.end()){
				first = i+1;
				second = j+1;
				third = ref[need] + 1;
				imp = false;
			}
			if(!imp) break;
			s.insert(val[j]);
			ref[val[j]] = j;
		}
	}

	//Write out
	if(imp)  cout << "IMPOSSIBLE" << "\n";
	else cout << first << " " << second << " " << third << "\n";
		
	//Return
	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: ACCEPTED

Test 13

Verdict: TIME LIMIT EXCEEDED

Test 14

Verdict: TIME LIMIT EXCEEDED

Test 15

Verdict: TIME LIMIT EXCEEDED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: TIME LIMIT EXCEEDED

Test 22

Verdict: TIME LIMIT EXCEEDED

Test 23

Verdict: ACCEPTED