代码：
#include <iostream>
#include <cstring>
#include <algorithm>
#include <stack>
#include <unordered_map>using namespace std;stack<int> num;
stack<char> op;void eval()//求值
{auto b = num.top(); num.pop();//第二个操作数auto a = num.top(); num.pop();//第一个操作数auto c = op.top(); op.pop();//运算符int x;//结果//计算结果if (c == '+') x = a + b;else if (c == '-') x = a - b;else if (c == '*') x = a * b;else x = a / b;num.push(x);//结果入栈
}int main()
{//优先级表unordered_map<char, int> pr{{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}};string str;//读入表达式cin >> str;for (int i = 0; i < str.size(); i ++ ){auto c = str[i];if (isdigit(c))//数字入栈{int x = 0, j = i;//计算数字while (j < str.size() && isdigit(str[j]))x = x * 10 + str[j ++ ] - '0';i = j - 1;num.push(x);//数字入栈}//左括号无优先级，直接入栈else if (c == '(') op.push(c);//左括号入栈//括号特殊，遇到左括号直接入栈，遇到右括号计算括号里面的else if (c == ')')//右括号{while (op.top() != '(') eval();//一直计算到左括号op.pop();//左括号出栈}else{//待入栈运算符优先级低，则先计算while (op.size() && op.top() != '(' && pr[op.top()] >= pr[c]) eval();op.push(c);//操作符入栈}}while (op.size()) eval();//剩余的进行计算cout << num.top() << endl;//输出结果return 0;
}
