3SUM

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    int n, x;
    cin >> n >> x;
    vector<pair<int, int>> arr(n);
    
    for (int i = 0; i < n; i++) {
        cin >> arr[i].first;
        arr[i].second = i + 1;
    }
    
    sort(arr.begin(), arr.end());
    
    for (int i = 0; i < n; i++) {
        int target = x - arr[i].first;
        int left = i + 1, right = n - 1;
        
        while (left < right) {
            int sum = arr[left].first + arr[right].first;
            
            if (sum == target) {
                cout << arr[i].second << " " << arr[left].second << " " << arr[right].second << endl;
                return 0;
            } else if (sum < target) {
                left++;
            } else {
                right--;
            }
        }
    }
    
    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: 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