中缀表达式求值

给定一个表达式，其中运算符仅包含 +,-,*,/（加 减 乘 整除），可能包含括号，请你求出表达式的最终值。

AC代码

#include <iostream>
#include <string>
#include <cstring>
#include <stack>
#include <algorithm>
#include <unordered_map>
using namespace std;

stack < int > num;
stack < char > op;

void work()
{
    auto b = num.top();num.pop();
    auto a = num.top();num.pop();
    auto c = op.top(); op.pop();
    int x;
    if (c == '+') x = a + b;
    else if (c == '-') x = a - b;
    else if (c == '*') x = a * b;
    else if (c == '/') x = a / b;
    else if (c == '^') x = a ^ b;
    num.push(x);
}

int main()
{
    unordered_map < char , int > g{{'+',1} ,{'-',1} ,{'*',2} ,{'/',2} ,{'^',3}};
    string st;
    cin >> st;
    for (int i = 0; i < st.size(); i++)
    {
        char c = st[i];
        if (isdigit(c))
        {
            int x = 0;
            while (i < st.size() && isdigit(st[i]))
                x = x * 10 + st[i ++ ] - '0';
            i--;
            num.push(x);
        }
        else if (c == '(') op.push(c);
        else if (c == ')')
        {
            while (op.top() != '(') work();
            op.pop();
        }
        else
        {
            while (op.size() && g[op.top()] >= g[c]) work();
            op.push(c);
        }
    }
    while (op.size()) work();
    cout << num.top();
    return 0;
}