3SUM

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

using namespace std;
typedef long long ll;

ll n;
vector<pair<ll, int>> arr;

pair<int, int> two_sum(int ignore, ll target) {
    int left = 0;
    int right = n - 1;

    while (left < right) {

        if (left == ignore) {
            left++;
            continue;
        }

        if (right == ignore) {
            right--;
            continue;
        }

        ll s = arr[left].first + arr[right].first;

        if (s < target)
            left++;
        else if (s > target)
            right--;
        else
            return {left, right};
    }
    return {-1, -1};
}

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

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

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

    for (int i = 0; i < n; i++) {
        auto res = two_sum(i, x - arr[i].first);

        if (res.first != -1) {
            cout << arr[i].second + 1 << " " << arr[res.first].second + 1 << " "
                 << arr[res.second].second + 1 << "\n";
            return 0;
        }
    }

    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: 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