678. Valid Parenthesis String

Given a string containing only three types of characters: '(', ')' and '', write a function to check whether this string is valid. We define the validity of a string by these rules:
Any left parenthesis '(' must have a corresponding right parenthesis ')'.
Any right parenthesis ')' must have a corresponding left parenthesis '('.
Left parenthesis '(' must go before the corresponding right parenthesis ')'.
'' could be treated as a single right parenthesis ')' or a single left parenthesis '(' or an empty string.
An empty string is also valid.

Solution：count

思路： 类似只有'(' 和 ')' 的情况，count分别加减1，看是否最终等于0. 这里存在*情况，所以变为scope[low, high]

Screen Shot 2017-12-25 at 14.39.51.png

Time Complexity: O(N) Space Complexity: O(1)

Solution Code：

class Solution {
    public boolean checkValidString(String s) {
        int low = 0;
        int high = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                low++;
                high++;
            } else if (s.charAt(i) == ')') {
                if (low > 0) {
                    low--;
                }
                high--;
            } else {
                if (low > 0) {
                    low--;
                }
                high++;
            }
            if (high < 0) {
                return false;
            }
        }
        return low == 0;
    }
}