電腦（無括号可以處理空格）

227. Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, ​

​+​

​​, ​

​-​

​​, ​

​*​

​​, ​

​/​

​ operators and empty spaces . The integer division should truncate toward zero.

Example 1:

Input: "3+2*2"

Output: 7

Example 2:

Input: " 3/2 "

Output: 1

Example 3:

class Solution {
public:
    int calculate(string s) {
        long Result = 0;
        int n = s.length();
        long num = 0;
        stack<int> S;
        char op = '+';
        for(int i = 0;i < n;i ++){
            if(s[i] >= '0' && s[i] <= '9'){
                num = num * 10 + s[i] - '0';
            }
            if((s[i] < '0' && s[i] != ' ' )|| i == n - 1){
                if(op == '+'){
                    S.push(num);
                }
                if(op == '-'){
                    S.push(-num);
                }
                if(op == '/'){
                    int tmp = S.top() / num;
                    S.pop();
                    S.push(tmp);
                }
                if(op == '*'){
                    int tmp = S.top() * num;
                    S.pop();
                    S.push(tmp);
                }
                op = s[i];
                num = 0;
            }
        }
        while(!S.empty()){
            Result += S.top();
            S.pop();
        }
        return Result;
    }
};