表达式求值

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

map<char, int> priority = {
    { '+', 1 },{ '-', 1 },{ '*', 2 },{ '/', 2 },{ '(', 0 }
};

vector<string> poststr;
stack<char> op;

void topoststr(string &str) {
   string num;
   for (int i = 0; i < str.size(); i++) {
      if (isdigit(str[i])) {
         num.clear();
         while (isdigit(str[i])) {
            num.push_back(str[i]);
            i++;
         }
         poststr.push_back(num);
         --i;
      }
      else if (str[i] == '(') {
         op.push(str[i]);
      }
      else if (str[i] == ')') {
         while (op.top() != '(') {
            string tmp;
            tmp = op.top();
            poststr.push_back(tmp);
            op.pop();
         }
         op.pop();
      }
      else {
         while (!op.empty() && priority[str[i]] <= priority[op.top()]) {
            string tmp;
            tmp = op.top();
            poststr.push_back(tmp);
            op.pop();
         }
         op.push(str[i]);
      }
   }
   while (!op.empty()) {
      string tmp;
      tmp.push_back(op.top());
      poststr.push_back(tmp);
      op.pop();
   }
}

int calculate(string &str) {
   stack<int> res;
   topoststr(str);
   for (unsigned int i = 0; i < poststr.size(); i++) {
      if (isdigit(poststr[i][0])) {
         res.push(atoi(poststr[i].c_str()));
      }
      else {
         int right = res.top();
         res.pop();
         if (!res.empty()) {
            int left = res.top();
            res.pop();
            switch (poststr[i][0]) {
            case '+': res.push(left + right); break;
            case '-': res.push(left - right); break;
            case '*': res.push(left * right); break;
            case '/': res.push(left / right); break;
            }
         }
      }
   }
   return res.top();
}

int main()
{
   string str;
   while (cin >> str)
      cout << calculate(str) << endl;
   return 0;
}

 

来源：https://blog.csdn.net/weixin_44100654/article/details/100085758