题解 | #木棍游戏#

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

int main() {
  std::cin.tie(nullptr)->sync_with_stdio(false);

  int n;
  std::cin >> n;
  std::vector<int> A(n);

  for (auto &i : A) {
    std::cin >> i;
  }

  double ans = 0.0;

  std::function<void(int, int, int, int)> DFS = 
[&](int a, int b, int c, int id) {

    if (a && b && c && a + b > c && a + c > b && b + c > a) {
      double p = a + b + c;
      p /= 2.;
      ans = std::max(ans, std::sqrt(p * (p - a) * (p - b) * (p - c)));
    }

    if (id > n) {
      return;
    }

    DFS(a, b, c + A[id], id + 1);
    DFS(a + A[id], b, c, id + 1);
    DFS(a, b + A[id], c, id + 1);
    DFS(a, b, c, id + 1);
  };

  DFS(0, 0, 0, 0);

  if (ans == 0) {
    std::cout << -1;
  } else {
    std::cout << std::fixed << std::setprecision(1) << ans;
  }

  return 0 ^ 0;
}