224 Basic Calculator

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

Note: Do not use the eval built-in library function.

 

class Solution {
public:
    int calculate(string s) {
        stack<int> calcst;
        calcst.push(1);
        s += ')';
        int sign = 1, num = 0, i = 0, ans = 0;
        for (i = 0; i < s.size(); i++){
            if (s[i] >= '0' && s[i] <= '9'){
                num = num * 10 + s[i] - '0';
            }else if (s[i] == '+' || s[i] == '-' || s[i] == ')'){
                ans += calcst.top() * sign * num;
                if (s[i] == ')') calcst.pop();
                sign = s[i] == '-' ? -1 : 1;
                num = 0;
            }else if (s[i] == '('){
                calcst.push(calcst.top() * sign);
                sign = 1;
            }
        }
        return ans;
    }
};