3SUM

#include <iostream>
#include <vector>
#include <array>
#include <string>
#include <algorithm>
#include <numeric>
#include <unordered_map>
#include <queue>
#include <climits>
#include <cmath>
 
//#define int long long
using namespace std;

void solve() {
    int n, x;
    cin >> n >> x;

    vector<pair<int, int>> nums(n);
    for (int i = 0; i < n; i++)
    {
        cin >> nums[i].first;
        nums[i].second = i+1;
    }
    
    sort(nums.begin(), nums.end());

    for (int i = 0; i < n - 2; i++)
    {
        int l = i+1;
        int r = n-1;
        while (l < r) {
            int c = nums[i].first+nums[l].first+nums[r].first;
            if (c == x)
            {
                cout << nums[i].second << " " << nums[l].second << " " << nums[r].second;
                return;
            }
            if (c < x)
                l++;
            else
                r--;
        }
    }

/*    
    unordered_set<int, int> passed;
    passed.reserve(n);
    for (int i = 0; i < nums.size(); i++)
    {
        passed.clear();
        for (int j = i+1; j < nums.size(); j++)
        {
            if (passed.find(x - nums[i] - nums[j]) != passed.end())
            {
                cout << i+1 << " " << j+1 << " " << passed[x-nums[i]-nums[j]] + 1;
                return;
            }
            passed[nums[j]] = j;
        }
    }
*/

    cout << "IMPOSSIBLE";
}


 
signed main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);

    int t = 1;
 
    //cin >> t;
 
    for (int i = 0; i < t; i++)
        solve();
 
    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