原题链接在这里：

题目：

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:

1. Any left parenthesis '(' must have a corresponding right parenthesis ')'.
2. Any right parenthesis ')' must have a corresponding left parenthesis '('.
3. Left parenthesis '(' must go before the corresponding right parenthesis ')'.
4. '*' could be treated as a single right parenthesis ')' or a single left parenthesis '(' or an empty string.
5. An empty string is also valid.

 

Example 1:

Input: "()"Output: True

Example 2:

Input: "(*)"Output: True 

Example 3:

Input: "(*))"Output: True 

Note:

1. The string size will be in the range [1, 100].

题解：

计数"("的数目. 遇到"("加一. 遇到")" 减一.

遇到"*". 可能分别对应"(", ")"和empty string 三种情况. 所以计数可能加一, 减一或者不变. 可以记录一个范围[lo, hi].

lo维持在0或以上的位置.

但若是hi都小于0, 说明确定的")"更多, 直接返回false.

否则看最后lo能否能停在0.

Time Complexity: O(s.length()). Space: O(1).

AC Java:

1 class Solution { 2     public boolean checkValidString(String s) { 3         if(s == null || s.length() == 0){ 4             return true; 5         } 6          7         int lo = 0; 8         int hi = 0; 9         for(int i = 0; i