224. Basic Calculator

class Solution {
    public int calculate(String s) {
        Deque<Integer> nums = new LinkedList<>();
        Deque<Character> ops = new LinkedList<>();
        int i = 0, n = s.length(), v = -1;
        char[] c = s.toCharArray();
        while(i < n)
        {
            char ch = c[i];
            if(ch == ' '){}   // skip white space
            else if(ch == '(')
            {
                int count = 1, j = i;
                while(count > 0)
                {
                    j++;
                    if(c[j] == '(') count++;
                    if(c[j] == ')') count--;
                }
                // now j points to position after )
                nums.push(calculate(s.substring(i + 1, j)));    // get direct result
                i = j;
            }
            else if('0' <= ch && ch <= '9')
            {
                if(v < 0) v = 0;
                v = v * 10 + ch - '0';
            }
            else    // ch is operator
            {
                if(v >= 0) nums.push(v);
                v = -1;
                ops.push(ch);
            }
            i++;
        }
        if(v >= 0) nums.push(v);

        // System.out.println(nums);
        // System.out.println(ops);

        while(ops.size() > 0)
        {
            int a = nums.pollLast(), b = nums.pollLast();
            char ch = ops.pollLast();
            if(ch == '+') nums.offerLast(a + b);
            else nums.offerLast(a - b);
            // System.out.println(nums);
        }

        // System.out.println(nums);
        return nums.pop();
    }
}