rt
代码:

#include <bits/stdc++.h>

using pis = std::pair<int, std::string>;

int main() {
  std::ios::sync_with_stdio(0);
  std::cin.tie(nullptr);

  std::vector<int> a(4, 0);
  for(int i = 0; i < 4; ++i) 
    std::cin >> a[i];
  auto calc = [&](int x, int y) -> std::vector<pis> {
    std::vector<pis> res;
    res.push_back({x+y, std::to_string(x) + "+" + std::to_string(y)
        + "=" + std::to_string(x+y) + "\n"});
    if(x > y) 
      res.push_back({x-y, std::to_string(x) + "-" + std::to_string(y)
          + "=" + std::to_string(x-y) + "\n"});
    res.push_back({x*y, std::to_string(x) + "*" + std::to_string(y)
        + "=" + std::to_string(x*y) + "\n"});
    if(y > x)
      res.push_back({y-x, std::to_string(y) + "-" + std::to_string(x)
          + "=" + std::to_string(y-x) + "\n"});
    if(x >= y && x % y == 0) 
      res.push_back({x/y, std::to_string(x) + "/" + std::to_string(y)
          + "=" + std::to_string(x/y) + "\n"});
    if(y >= x && y % x == 0) 
      res.push_back({y/x, std::to_string(y) + "/" + std::to_string(x)
          + "=" + std::to_string(y/x) + "\n"});
    return res;
  };
  auto f = [&](auto &f) -> std::vector<pis> {
    std::vector<pis> res;
    if(a.size() == 1) {
      res.push_back({a[0], ""});
      return res;
    }
    for(int i = 0; i < a.size(); ++i) {
      int tmp = a[i];
      a.erase(a.begin()+i);
      auto f_res = f(f);
      a.insert(a.begin()+i, tmp);
      for(auto x: f_res) {
        auto calc_res = calc(x.first, tmp);
        for(auto y: calc_res) {
          res.push_back({y.first, x.second + y.second});
        }
      }
    }
    return res;
  };
  auto ans = f(f);
  for(auto x: ans) {
    if(x.first == 24) {
      std::cout << x.second;
      return 0;
    }
  }
  auto g = [&]() -> std::vector<pis> {
    std::vector<pis> res;
    std::vector<int> tmp(2, 0);
    for(int i = 0; i < 3; ++i) {
      for(int j = i+1; j < 4; ++j) {
        tmp[0] = a[i];
        tmp[1] = a[j];
        a.erase(a.begin()+j);
        a.erase(a.begin()+i);
        auto f_res = f(f);
        a = tmp;
        auto f2_res = f(f);
        for(auto k: f_res) {
          for(auto l: f2_res) {
            auto calc_res = calc(k.first, l.first);
            for(auto y: calc_res) {
              res.push_back({y.first, k.second+l.second+y.second});
            }
          }
        }
      }
    }
    return res;
  };
  ans = g();
  for(auto x: ans) {
    if(x.first == 24) {
      std::cout << x.second;
      return 0;
    }
  }
  std::cout << "No answer!";
}

WA 了三个点，用红名权限下载了 #6 的数据:
输入:

3 4 5 6

正确输出:

4-3=1
5-1=4
6*4=24

我的输出:

5-4=1
1+3=4
4*6=24

感觉也没有问题啊，是我的问题还是 Special Judge 不行？麻烦 dalao 帮忙看看...