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 .

Example 1:

Input: "1 + 1"
Output: 2

Example 2:

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

Example 3:

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

Note:

• You may assume that the given expression is always valid.
• Do not use the eval built-in library function.

class Solution {
    public int calculate(String s) {
        Stack<Integer> stack = new Stack<>();
        stack.push(0);                          // Always keep most recent sum at top
        for (int i = 0, sign = 1; i < s.length(); i++) {
            if (Character.isDigit(s.charAt(i))) {
                int num = s.charAt(i) - '0';    // Be aware of outer loop boundary and i++
                for (; i < s.length() - 1 && Character.isDigit(s.charAt(i + 1)); i++) {
                    num = num * 10 + (s.charAt(i + 1) - '0');
                }
                stack.push(stack.pop() + sign * num);//pre_sum + cur_number
            } else if (s.charAt(i) == '+') {
                sign = 1;
            } else if (s.charAt(i) == '-') {
                sign = -1;
            } else if (s.charAt(i) == '(') {
                stack.push(sign);//store outer_sign
                stack.push(0);//like what we did at the start, 0 is a pre_sum
                sign = 1;//by default sign == 1
            } else if (s.charAt(i) == ')') {    // Update last sum = current sum * sign
                stack.push(stack.pop() * stack.pop() + stack.pop());//(local_sum * outer_sign + pre_sum)
            } /* else whitespace*/
        }
        return stack.pop();
    }
}