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.

Solution:

public class Solution {
    public int calculate(String s) {
        Stack<Integer> stack = new Stack<>();
        int result = 0;
        int sign = 1;
        for(int i=0; i<s.length(); i++){
            char c = s.charAt(i);
            if(c>='0' && c<='9'){
                int num = s.charAt(i) - '0';
                while(i+1<s.length() && s.charAt(i+1)>='0' && s.charAt(i+1)<='9'){
                    num = num * 10 + (s.charAt(i+1) - '0');
                    i++;
                }
                result += num * sign;
                sign = 1;
            } else if(c=='+'){
                sign = 1;
            } else if(c=='-'){
                sign = -1;
            } else if(c=='('){
                stack.push(result);
                stack.push(sign);
                result = 0;
                sign = 1;
            } else if(c==')'){
                result =  stack.pop() * result + stack.pop();
            }
        }
        return result;
    }
}