3SUM

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

using namespace std;
typedef long long ll;

bool compare(const pair<int, int> &i, const pair<int, int> &j) {
    return i.first < j.first;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

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

    std::sort(arr.begin(), arr.end(), compare);

    for (int i = 0; i < n; i++) {
        for (int j = i + 1; j < n; j++) {

            ll need = x - arr[i].first - arr[j].first;

            if (need - arr[j + 1].first < 0) {
                cout << "IMPOSSIBLE\n";
                return 0;
            }

            int a = j + 1, b = n - 1;
            while (a <= b) {
                int k = (a + b) / 2;
                if (arr[k].first == need) {
                    cout << arr[i].second + 1 << " " << arr[j].second + 1 << " "
                         << arr[k].second + 1 << "\n";
                    return 0;
                }
                if (arr[k].first > need)
                    b = k - 1;
                else
                    a = k + 1;
            }
        }
    }

    cout << "IMPOSSIBLE\n";
    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: WRONG ANSWER

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: WRONG ANSWER

Test 17

Verdict: WRONG ANSWER

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: WRONG ANSWER

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED