题解 | #计算表达式#

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

using namespace std;

double calculate(char opera, double leftNum, double rightNum);


map<char, int> priority = {
        {'$', 0},
        {'+', 1},
        {'-', 1},
        {'*', 2},
        {'/', 2}
};

/**
 * 计算表达式--上海交通大学
 * @return
 */
int main() {
    string str;
    cin >> str;
    str += "#";
    stack<char> operaStack;
    stack<double> numStack;
    string num = "";
    for (int i = 0; i < str.size(); ++i) {
        char cur = str[i];
        if (cur == '#') {
            if (num != "") {
                numStack.push(stod(num));
                num = "";
            }
            while (!operaStack.empty()) {
                char opera = operaStack.top();
                operaStack.pop();
                double rightNum = numStack.top();
                numStack.pop();
                double leftNum = numStack.top();
                numStack.pop();
                numStack.push(calculate(opera, leftNum, rightNum));
            }
        } else if ('0' <= cur && cur <= '9') {
            num += cur;
        } else {
            if (num != "") {
                numStack.push(stod(num));
                num = "";
            }
            while (!operaStack.empty() && priority[operaStack.top()] >= priority[cur]) {
                char opera = operaStack.top();
                operaStack.pop();
                double rightNum = numStack.top();
                numStack.pop();
                double leftNum = numStack.top();
                numStack.pop();
                numStack.push(calculate(opera, leftNum, rightNum));
            }
            operaStack.push(cur);
        }
    }
    cout << numStack.top() << endl;


    return 0;
}

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;
}