3SUM

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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    long long x;
    cin >> n >> x;
    if(n <3){
        cout << "IMPOSSIBLE";
        return 0;
    }
    // Pair of value and original index (1-based)
    vector<pair<long long, int>> a(n);
    for(int i=0;i<n;i++){
        cin >> a[i].first;
        a[i].second = i+1;
    }
    sort(a.begin(), a.end());
    for(int i=0;i<n-2;i++){
        long long target = x - a[i].first;
        int left = i+1;
        int right = n-1;
        while(left < right){
            long long current_sum = a[left].first + a[right].first;
            if(current_sum == target){
                int idx1 = a[i].second;
                int idx2 = a[left].second;
                int idx3 = a[right].second;
                vector<int> indices = {idx1, idx2, idx3};
                sort(indices.begin(), indices.end());
                cout << indices[0] << " " << indices[1] << " " << indices[2];
                return 0;
            }
            else if(current_sum < target){
                left++;
            }
            else{
                right--;
            }
        }
    }
    cout << "IMPOSSIBLE";
}

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