3SUM

#include <iostream>
#include <vector>
#include <algorithm>
#include <unordered_map>

using namespace std;

int main(){
    long long n, x;
    cin >> n >> x;
    vector<pair<int,int>> arr(n+1);


    for (long long i=1;i<=n;i++){
        cin >> arr[i].first;
        arr[i].second=i;
    }

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

    for (int i=1;i<n-1;i++){
        int l=i+1, r=n;
        while (l<r){
            if (arr[i].first+arr[l].first+arr[r].first==x){
                cout << arr[i].second << ' ' << arr[l].second << ' ' << arr[r].second << '\n'; 
                return 0;
            } else if (arr[i].first+arr[l].first+arr[r].first>x){
                r--;
            } else {
                l++;
            }
        }
    }
    cout << "IMPOSSIBLE\n";
    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