Shared code

#include <iostream>
#include <unordered_map>
#include <vector>
constexpr int N = 5000;
 
int a[N];
 
int main() {
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(nullptr);
 
  int n, x;
  std::cin >> n >> x;
  for (int i = 0; i < n; ++i) {
    std::cin >> a[i];
  }
 
  std::unordered_map<int, int> pos;
  pos.reserve(n * 10);
  for (int i = 0; i < n; ++i) {
    pos[a[i]] = i;
  }
 
  for (int i = 0; i < n; ++i) {
    for (int j = i + 1; j < n; ++j) {
      auto it = pos.find(x - a[i] - a[j]);
      if (it == pos.end() or it->second <= j) {
        continue;
      }
      std::cout << i + 1 << ' ' << j + 1 << ' ' << it->second + 1 << '\n';
      return 0;
    }
  }
  std::cout << "IMPOSSIBLE\n";
  return 0;
}