题解 | 计算表达式

#include <iostream>
#include <stdio.h>
#include <map>
#include <stack>
#include <string>
#include <string.h>
using namespace std;

int main() {
    char str[1000] = { 0 };
    map<char, int> priority = {
        {'\0', 0},
        {'-', 1}, {'+', 1},
        {'*', 2}, {'/', 2}
    };//用来记录运算的优先级
    while (scanf("%s", str) != EOF) {
        string numStr = "";
        stack<double> numStack;
        stack<char> opStack;
        for (int i = 0;; i++) {
            if (str[i] >= '0' && str[i] <= '9') {
                numStr.push_back(str[i]);
            } else {
                if (!numStr.empty()) {
                    double num = stod(numStr);
                    numStr = "";
                    numStack.push(num);
                }
                // 什么时候弹栈 ？ 栈非空 &&新op的优先级 不高于栈顶的优先级
                // 循环弹栈或者计算
                while (!opStack.empty() && priority[str[i]] <= priority[opStack.top()]) {
                    double rhs = numStack.top();
                    numStack.pop();
                    double lhs = numStack.top();
                    numStack.pop();
                    char curOp = opStack.top();
                    opStack.pop();
                    if (curOp == '+') {
                        numStack.push(lhs + rhs);
                    } else if (curOp == '*') {
                        numStack.push(lhs * rhs);
                    } else if (curOp == '/') {
                        numStack.push(lhs / rhs);
                    } else if (curOp == '-') {
                        numStack.push(lhs - rhs);
                    }
                }
                //离开了循环 栈为空或者新op优先级高于栈顶
                if (str[i] == '\0') {
                    printf("%d\n", (int)numStack.top());
                    break;
                } else {
                    opStack.push(str[i]);
                }
            }

        }
    }
    return 0;
}