题解 | #计算表达式#

#include <iostream>
#include <stack>
using namespace std;

int Priority(char ch) {
    if (ch == '#') {
        return 0;
    } else if (ch == '$') {
        return 1;
    } else if (ch == '+' || ch == '-') {
        return 2;
    } else {
        return 3;
    }
}
double opeNum(double a, double b, char ch) {
    if (ch == '+') {
        return a + b;
    } else if (ch == '-') {
        return a - b;
    } else if (ch == '*') {
        return a * b;
    } else {
        return a / b;
    }
}
int main() {
    string str;
    while (cin >> str) {
        stack<double> num;
        stack<char> ope;
        ope.push('#');
        str += '$';
        int i=0;
        while(i<str.size()){
            if(isdigit(str[i])){
                double number = 0;
                while (isdigit(str[i])) {
                    number = number * 10 + str[i] - '0';
                    i++;
                } 
                num.push(number);  
            } else {
                char ch=ope.top();
                if (Priority(str[i]) > Priority(ch)) {
                    ope.push(str[i]);
                    i++;
                } else {
                    double b = num.top();
                    num.pop();
                    double a = num.top();
                    num.pop();
                    num.push(opeNum(a, b, ch));
                    ope.pop();
                }
            }
        }
        cout<<num.top()<<endl;
    }
}