四则运算实现

实现简单的加减乘除四则运算

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

/**
 * 实现简单的四则运算器，支持加、减、乘、除四种运算。
 */

int calculate(string s){
    stack<int> stk;
    int num = 0;
    char sign = '+';
    for(int i = 0; i < s.size(); ++i) {
        char c = s[i];
        if (isdigit(c)) {
            num = num * 10 + c - '0';
        }
        if (!isdigit(c) || i == s.size() - 1) {
            switch (sign) {
                int pre;
                case '+': stk.push(num); break;
                case '-': stk.push(-num); break;
                case '*': 
                    pre = stk.top();
                    stk.pop();
                    stk.push(pre * num); 
                    break;
                case '/': 
                    pre = stk.top();
                    stk.pop();
                    stk.push(pre / num); 
                    break;
            }
            sign = c;
            num = 0;
        }
    }
    int res = 0;
    while (!stk.empty()) {
        res += stk.top();
        stk.pop();
    }

    return res;
}

int main() {
    std::string s = "20+30*40-80";
    int res = calculate(s);
    cout << "res:" << res << endl;
    system("pause");
    return 0;
}

参考：https://mp.weixin.qq.com/s/ds0guq9gPTLIHLEQnFxZVQ