leetcode 224. Basic Calculator ---------- java

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.

public class Solution {
   public int calculate(String s) {

        Stack<Integer> stack = new Stack<Integer>();
        int result = 0;
        int number = 0;
        int sign = 1;
        for(int i = 0; i < s.length(); i++){
            char c = s.charAt(i);
            if(Character.isDigit(c)){
                number = 10 * number + (int)(c - '0');
            }else if(c == '+'){
                result += sign * number;
                number = 0;
                sign = 1;
            }else if(c == '-'){
                result += sign * number;
                number = 0;
                sign = -1;
            }else if(c == '('){
                //we push the result first, then sign;
                stack.push(result);
                stack.push(sign);
                //reset the sign and result for the value in the parenthesis
                sign = 1;
                result = 0;
            }else if(c == ')'){
                result += sign * number;
                number = 0;
                result *= stack.pop();    //stack.pop() is the sign before the parenthesis
                result += stack.pop();   //stack.pop() now is the result calculated before the parenthesis

            }
        }
        if(number != 0) result += sign * number;
        return result;
    }
}