921. Minimum Add to Make Parentheses Valid

/*
Time Complexity: O(N)
Space Complexity: O(1)
N = length of the string
*/
class Solution {
    public int minAddToMakeValid(String S) {
        if (S == null || S.length() == 0) {
            return 0;
        }

        int result = 0;
        int count = 0;
        for (char s : S.toCharArray()) {
            count += s == '(' ? 1 : -1;
            if (count < 0) {
                result++;
                count = 0;
            }
        }

        return result + count;
    }
}