[LeetCode] 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 .

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: Push the current result & sign into stack when encountering parentheses. Pop them up when the end of the parentheses.

class Solution {
    public int calculate(String s) {
        if (s == null || s.isEmpty()) return 0;
        
        Stack<Integer> stack = new Stack();
        int sign = 1;
        int number = 0;
        int sum = 0;
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (Character.isDigit(ch)) {
                number = number * 10 + ch - '0';
            } else if (ch == '+') {
                sum  = sum + sign * number;
                sign = 1;
                number = 0;
            } else if (ch == '-') {
                sum  = sum + sign * number;
                sign = -1;
                number = 0;
            } else if (ch == '(') {
                stack.push(sum);
                stack.push(sign);
                sum = 0;
                sign = 1;
            } else if (ch == ')') {
                sum  = sum + sign * number;
                
                sum *= stack.pop();
                sum += stack.pop();
                number = 0;
            }
        }
        sum  = sum + sign * number;
        return sum;
    }
}