题解 | #简单计算器#

#include <iostream>
#include <cstdio>
#include <string>
#include <algorithm>
#include <stack>
#include <map>

using namespace std;

/**
 * 中缀表达式转后缀表达式（逆波兰式）
 * @param formula
 * @return
 */
string getRPN(string formula);

/**
 * 后缀表达式求值
 * @param rpnFormula
 * @return
 */
double getRPNRes(string rpnFormula);

/**
 * 计算运算符的操作结果
 * @param opera 运算符
 * @param leftNum 左操作数
 * @param rightNum 右操作数
 * @return
 */
double calculate(char opera, double leftNum, double rightNum);

/**
 * map存放运算符的优先级
 */
map<char, int> priority = {
    {'$', 0},   //结尾符$--1
    {'+', 1},   //加--1
    {'-', 1},   //减--1
    {'*', 2},   //乘--2
    {'/', 2}    //除--2
};

/**
 * 简单计算器--浙江大学
 * @return
 */
int main() {
    string str;
    while (getline(cin, str)) {
        if (str == "0") {
            break;
        }
        //在字符串的结尾添加"$"
        str += "$";
        string formula = getRPN(str);
        double res = getRPNRes(formula);
        printf("%.2f\n", res);
        //cout << formula << endl;
        //cout << res << endl;
    }

    return 0;
}

string getRPN(string formula) {
    stack<char> operaStack;
    string res = "";
    string num = "";
    for (int i = 0; i < formula.size(); ++i) {
        char character = formula[i];
        if (character == '$') {
            //来到最后一个字符$
            //将num加入道后缀表达式中
            res += num;
            res += " ";
            //将字符栈中剩余的运算符加入道后缀表达式中
            while (!operaStack.empty()) {
                res += operaStack.top();
                operaStack.pop();
                res += " ";
            }

        } else if (character == ' ') {
            //空格, 则表示一个单位已经扫描完
            if (num != "") {
                res += num;
                res += " ";
                num = "";
            }
        } else if ('0' <= character && character <= '9' || character == '.') {
            //数字
            num.push_back(character);
        } else if (('a' <= character && character <= 'z') || ('A' <= character &&
                   character <= 'Z')) {
            res += character;
            res += " ";
        } else if (character == '(') {
            //"(", 则直接入栈
            operaStack.push(character);
        } else if (character == ')') {
            //")", 则弹出栈内的运算符，并加入到后缀表达式中，直至弹出"("
            while (!operaStack.empty() && operaStack.top() != '(') {
                res += operaStack.top();
                operaStack.pop();
                res += " ";
            }
        } else {
            /*
             * 运算符
             * 依次弹出所有优先级高于或者等于当前运算符的运算符，并将其加入到后缀表达式中。
             */
            while (!operaStack.empty() &&
                    priority[operaStack.top()] >= priority[character]) {
                res += operaStack.top();
                operaStack.pop();
                res += " ";
            }
            //当前运算符入栈
            operaStack.push(character);
        }
    }
    return res;
}

double getRPNRes(string formula) {
    stack<double> numStack;
    string num = "";
    for (int i = 0; i < formula.size(); ++i) {
        char character = formula[i];
        if (character == ' ') {
            if (num != "") {
                numStack.push(stod(num));
                num = "";
            }
        } else if ('0' <= character && character <= '9' || character == '.') {
            num.push_back(character);
        } else {
            double rightNum = numStack.top();
            numStack.pop();
            double leftNum = numStack.top();
            numStack.pop();
            numStack.push(calculate(character, leftNum, rightNum));
        }
    }

    if (numStack.size() == 1) {
        return numStack.top();
    } else {
        return -1;
    }
}

double calculate(char opera, double leftNum, double rightNum) {
    double res = 0;
    switch (opera) {
        case '+':
            res = leftNum + rightNum;
            break;
        case '-':
            res = leftNum - rightNum;
            break;
        case '*':
            res = leftNum * rightNum;
            break;
        case '/':
            res = leftNum / rightNum;
            break;
        default:
            break;
    }
    return res;
}