LeetCode 0224 - Basic Calculator

Input: "1 + 1"
Output: 2

Input: " 2-1 + 2 "
Output: 3

Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23

class Solution {
public:
    int calculate(string s) {
        stack<int> sum_stack;
        int result = 0;
        int sign = 1;
        for(int i = 0; s[i]; i++) {
            if(s[i] >= '0' && s[i] <= '9') {
                int number = s[i] - '0';
                while(s[i+1] && s[i+1] >= '0' && s[i+1] <= '9') {
                    i++;
                    number = number * 10 + s[i] - '0';
                }
                result += number * sign;
            } else if(s[i] == '+') {
                sign = 1;
            } else if(s[i] == '-') {
                sign = -1;
            } else if(s[i] == '(') {
                sum_stack.push(result);
                sum_stack.push(sign);
                result = 0;
                sign = 1;
            } else if(s[i] == ')') {
                auto current_sign = sum_stack.top();
                sum_stack.pop();
                auto current_result = sum_stack.top();
                sum_stack.pop();
                result = current_sign * result + current_result;
            }
        }
        return result;
    }
};