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) {
        if (s.empty())
            return 0;
        int res = 0;
        int sign = 1;
        stack<int> stack;
        int len = int(s.length());
        for (int i = 0; i < len; i++) {
            char c = s[i];
            if (isdigit(c)) {
                int cur = c - '0';
                while (i + 1 < len && isdigit(s[i + 1])) {
                    cur = cur * 10 + s[i+1] - '0';
                    i++;
                }
                res += cur * sign;
            } else if (c == '+') {
                sign = 1;
            } else if (c == '-') {
                sign = -1;
            } else if (c == '(') {
                stack.push(res);
                stack.push(sign);
                res = 0;
                sign = 1;
            } else if (c == ')') {
                res = stack.top() * res;
                stack.pop();
                res += stack.top();
                stack.pop();
                sign = 1;
            }
        }
        return res;
    }
};