题解 | #计算表达式#

#include <iostream>
#include <stack>
#include <unordered_map>

using namespace std;

stack<char> op;
stack<double> num;

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

int main()
{
    string s;
    cin >> s;

    unordered_map<char, int> pr{{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}};
    for(int i = 0; i < s.size(); i ++ ) 
    {
        if(isdigit(s[i]))
        {
            int j = i;
            double x = 0;
            while(j < s.size() && isdigit(s[j]))
                x = x * 10 + s[j ++ ] - '0';
            num.push(x);
            i = j - 1;
        }
        else if(s[i] == '(') op.push(s[i]);
        else if(s[i] == ')') 
        {
            while(op.top() != '(') eval();
            op.pop(); 
        }
        else
        {
            while(op.size() && op.top() != '(' && pr[op.top()] >= pr[s[i]])
                eval();
            op.push(s[i]);
        }
    }
    while(op.size()) eval();
    cout << num.top() << endl;
    return 0;
}