题解 | #简单计算器#

#include <iostream>
#include<stack>
//参考答案解法，磕磕绊绊，还不熟练
using namespace std;
int priority(char c) {
    if (c == '#') { //运算符栈
        return 0;
    } else if (c == '$') {
        return 1;
    } else if (c == '+' || c == '-') {
        return 2;
    } else {
        return 3;
    }
}
double getNumber(string str, int& index) { //获得索引
    double number = 0;
    while (isdigit(str[index])) { //判断是否是数字
        number = number * 10 + str[index] - '0';
        index++;
    }
    return number;
}
double Calculate(char operSign, double operNum1, double operNum2) {
    double resultNum = 0;
    if (operSign == '+') {
        resultNum = operNum1 + operNum2;
    } else if (operSign == '-') {
        resultNum = operNum1 - operNum2;
    } else if (operSign == '*') {
        resultNum = operNum1 * operNum2;
    } else if (operSign == '/') {
        resultNum = operNum1 / operNum2;
    }
    return resultNum;
}
int main() {
    string str;
    while (getline(cin, str)) {
        if (str == "0") break; //双引号，0就退出
        stack<double> data;
        stack<char> oper;
        int index = 0;
        str += '$';
        oper.push('#');
        while (index < str.size()) {
            if (isdigit(str[index])) {
                data.push(getNumber(str, index));

            } else if (str[index] == ' ') {
                index++;
            } else {
                if (priority(oper.top()) < priority(str[index])) {
                    oper.push(str[index]);
                    index++;
                } else {
                    double x = data.top();
                    data.pop();
                    double y = data.top();
                    data.pop();
                    data.push(Calculate(oper.top(), y, x));
                    oper.pop();
                }
            }

        }
        printf("%.2f\n", data.top());

    }
    return 0;
}