3SUM

#include <algorithm>
#include <iostream>
#include <queue>
#include <vector>
#define ll long long int
using namespace std;



void solve(vector<ll>& arr, ll X, int N)
{

    vector<vector<ll> > vec(N, vector<ll>(2));
    for (int i = 0; i < N; i++) {
        vec[i][0] = arr[i];
        vec[i][1] = i + 1;
    }

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

    for (ll ptr1 = 0; ptr1 < N - 2; ptr1++) {

        ll ptr2 = ptr1 + 1;
        ll ptr3 = N - 1;

        while (ptr2 < ptr3) {
            ll currentSum = vec[ptr1][0] + vec[ptr2][0]
                            + vec[ptr3][0];

            if (currentSum == X) {
                cout << vec[ptr1][1] << " " << vec[ptr2][1]
                     << " " << vec[ptr3][1] << endl;
                return;
            }
  
            else if (currentSum > X) {
                ptr3--;
            }
       
            else if (currentSum < X) {
                ptr2++;
            }
        }
    }

    cout << "IMPOSSIBLE";
}



int main() 
{ 
    int n;
    ll t;
    cin >> n >> t;
    vector<ll> arr(n);
    for (int i = 0; i < n; i++)
    {
        cin >> arr[i];
    }
    
    solve(arr, t, arr.size()); 
}

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