3SUM

#include <bits/stdc++.h>

using namespace std;

#define debug(x) cerr << #x << ": " << x << endl;
#define ll long long
#define ull unsigned long long
#define vi vector<int>

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

// Function to find a triplet with a given sum in an array
void find3Numbers(vector<ll>& arr, ll sum)
{
    int n = arr.size();

    vector<pair<ll,size_t>> val_and_pos (n);
    for (int i = 0; i < n; i++)
    {
        val_and_pos[i] = {arr[i], i};
    }
    
    
    // Sort the elements
    sort(val_and_pos.begin(), val_and_pos.end());

    // Fix the first element one by one 
    // and find the other two elements
    for (int i = 0; i < n - 2; i++) {

        // To find the other two elements, start two index
        // variables from two corners of the array and move
        // them toward each other
        int l = i + 1;  // index of the first element
                        // in the remaining elements
        int r = n - 1;  // index of the last element

        while (l < r) {
            int curr_sum = val_and_pos[i].first
                         + val_and_pos[l].first
                         + val_and_pos[r].first;
            if (curr_sum == sum) {
                cout << val_and_pos[i].second + 1 << " "
                    << val_and_pos[l].second + 1 << " " << val_and_pos[r].second + 1 << endl;
                return;
            } else if (curr_sum < sum)
                l++;
            else // curr_sum > sum
                r--;
        }
    }
  
    // If we reach here, then no triplet was found
    cout << "IMPOSSIBLE" << endl;
}

int main() {
    int n;
    ll x;
    cin >> n >> x;

    vector<ll> arr (n);
    for (int i = 0; i < n; i++)
    {
        cin >> arr[i];
    }

    find3Numbers(arr, x);

    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