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];
	map<ll,vl> ref;
	ll value;
	REP(i,0,n){
	       	cin >> value;
		val[i] = value;
		ref[value].PB(i);
	}

	//Main part
	bool imp = true;
	ll res[3];
	sort(val,val+n);
	REP(i,0,n){
		if(!imp) break;
		if(i>0 && val[i]==val[i-1]) continue;
		ll need = sum - val[i];
		ll l = i+1;
		ll r = n-1;
		while(l<r){
			if(val[l]+val[r] == need){
				res[0]=ref[val[i]][0];
				int ind2 = 0;
				if(val[i]==val[l]) ind2 = 1;
				res[1]=ref[val[l]][ind2];
				int ind3 = 0;
				if(val[i]==val[r]) ind3 = 2;
				else if(val[l]==val[r]) ind3 = 1;
				res[2]=ref[val[r]][ind3];
				imp = false;
				break;	
			}
			else if(val[l]+val[r]<need) l++;
			else r --;
		}
	}

	//Write out
	if(imp)  cout << "IMPOSSIBLE" << "\n";
	else{
		sort(res,res+3);
		REP(i,0,3) cout << res[i]+1 << " ";
		cout << endl;
	}
		
	//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: 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