Basic Calculator with parenthesis

Leetcode 224 加上parenthesis， 例如"2+((8+2)+(3-999))"，返回计算结果。

class Solution {
    public int calculate(String s) {
        Deque<Character> queue = new LinkedList<>();
        for(int i = 0; i< s.length(); i++)
        {
            if(s.charAt(i) != ' ')
            {
                queue.addLast(s.charAt(i));
            }
        }
        return helper(queue);
    }
    
    public int helper(Deque<Character> queue)
    {
        Deque<Integer> stack = new LinkedList<>();
        int num = 0;
        char preSign = '+';
        while(queue.size() > 0)
        {
            char current = queue.removeFirst();
            
            if(isNumber(current))
            {
                num = num * 10 + (current - '0');
            }
            if(current == '(')
            {
                num = helper(queue);
            }
            if(!isNumber(current) || queue.size() == 0)
            {
                switch(preSign)
                {
                    case '+': stack.push(num); break;
                    case '-': stack.push(-num); break;
                }
                preSign = current;
                num = 0;
            }
            if(current == ')') break;
        }
        
        int res = 0;
        while(stack.size() > 0)
        {
            res += stack.removeLast();
        }
        return res;
        
    }
    
    public boolean isNumber(char c)
    {
        return c >= '0' && c <= '9';
    }
}