3SUM

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
    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());

    auto it = lower_bound(nums.begin(), nums.end(), make_pair(x, 0));

    vector<int> result(3, -1);

    bool found = false;
    auto it1 = nums.begin();
    while (it1 != it and not found) {
        auto it2 = it1 + 1;
        while (it2 != it and not found)
        {
            if ((*it1).first + (*it2).first < x) {
                auto it3 = lower_bound(it2 + 1, it, make_pair(x - (*it1).first - (*it2).first, 0));
                if (it3 != it && (*it1).first + (*it2).first + (*it3).first == x) {
                    result[0] = (*it1).second;
                    result[1] = (*it2).second;
                    result[2] = (*it3).second;
                    found = true;
                }
            }
            ++it2;
        }
        ++it1;
    }

    if (found) {
        cout << result[0] << " " << result[1] << " " << result[2] << "\n";
    } else {
        cout << "IMPOSSIBLE" << endl;
    }   
}

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