题解 | #计算表达式#

#include <iostream>
#include <string>
#include <stack>
#include <map>

using namespace std;

int priority(string c)
{
  if (c == "+" || c == "-")
    return 1;
  if (c == "*" || c == "/")
    return 2;
  return 0;
}
double caculator(double x, double y, string c)
{
  if (c == "+")
  {
    return x + y;
  }
  else if (c == "-")
  {
    return x - y;
  }
  else if (c == "*")
  {
    return x * y;
  }
  else
  {
    return x / y;
  }
}

int main()
{

  // 6/2+3+3*4
  string input_str;
  while (getline(cin, input_str))
  {
    input_str = input_str + "#";
    string temp_str = "";
    stack<double> nums_stack;
    stack<string> fuhao_stack;
    for (int i = 0; i < input_str.size(); i++)
    {
      if (!isdigit(input_str[i]))
      {
        nums_stack.push(stod(temp_str));
        temp_str = "";
        while (!fuhao_stack.empty() && priority(fuhao_stack.top()) >= priority(string(1,input_str[i])))
        {
          double a = nums_stack.top();
          nums_stack.pop();
          double b = nums_stack.top();
          nums_stack.pop();
          string fuhao = fuhao_stack.top();
          fuhao_stack.pop();
          double caculate = caculator(b, a, fuhao);
          nums_stack.push(caculate);
        }
        fuhao_stack.push(string(1,input_str[i]));        
      }
      else
      {
        temp_str = temp_str + input_str[i];
      }
    }
    double result = nums_stack.top();
    printf("%.0f", result);
  }
}
// 64 位输出请用 printf("%lld")