3SUM

#include <iostream>
#include <vector>
#include <unordered_map>

using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    long long t;
    if (!(cin >> n >> t)) return 0;

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

    unordered_map<long long, int> single;
    unordered_map<long long, pair<int, int>> double_map;

    for (int i = 0; i < n; i++) {
        long long x = arr[i];

        if (double_map.count(x)) {
            cout << double_map[x].first + 1 << " " 
                 << double_map[x].second + 1 << " " 
                 << i + 1 << "\n";
            return 0;
        }

        for (auto const& [needed_single, idx_single] : single) {
            long long val = arr[idx_single];
            if (t - val - x > 0) {
                double_map[t - val - x] = {idx_single, i};
            }
        }

        if (t - x > 0 && !single.count(t - x)) {
            single[t - x] = i;
        }
    }

    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: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: TIME LIMIT EXCEEDED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: TIME LIMIT EXCEEDED

Test 22

Verdict: TIME LIMIT EXCEEDED

Test 23

Verdict: ACCEPTED