3SUM

#include <bits/stdc++.h>
using namespace std;

#define I_2D(row, col, width) ((row) * (width) + (col))
#define PRINT_ARR(arr, n)                                                      \
  do {                                                                         \
    for (int i = 0; i < n; i++) {                                              \
      cout << arr[i] << " ";                                                   \
    }                                                                          \
    cout << "\n";                                                              \
  } while (0)
#define PRINT_VEC_ARR(v, n)                                                    \
  do {                                                                         \
    for (int i = 0; i < n; i++) {                                              \
      cout << i << ": ";                                                       \
      for (auto item : v[i]) {                                                 \
        cout << item << " ";                                                   \
      }                                                                        \
      cout << endl;                                                            \
    }                                                                          \
  } while (0)

typedef long long ll;

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  ll n, x;
  cin >> n >> x;

  vector<ll> arr(n, 0);
  unordered_map<ll, vector<int>> mapping;

  for (int i = 0; i < n; i++) {
    cin >> arr[i];
    mapping[arr[i]].push_back(i);
  }

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

  ll sum;
  for (int i = 0; i < n - 2; i++) {
    int f = i + 1;
    int s = n - 1;

    while (f < s) {
      sum = arr[i] + arr[f] + arr[s];
      if (sum == x) {
        cout << mapping[arr[i]].back() + 1 << " ";
        mapping[arr[i]].pop_back();
        cout << mapping[arr[f]].back() + 1 << " ";
        mapping[arr[f]].pop_back();
        cout << mapping[arr[s]].back() + 1 << endl;
        mapping[arr[s]].pop_back();
        return 0;
      } else if (sum < x) {
        f++;
      } else {
        s--;
      }
    }
  }
  cout << "IMPOSSIBLE" << endl;
  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