Given a string s containing only three types of characters: '(', ')' and '*', return true if s is valid.

The following rules define a valid string:

• 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 "".

 

Example 1:

Input: s = "()"
Output: true

Example 2:

Input: s = "(*)"
Output: true

Example 3:

Input: s = "(*))"
Output: true

分析：
我们用lower and upper来表示“(”可能出现的最小次数和最大次数。但是如果lower小于0，那是一种invalid case,在upper大于0的这种情况下，我们可以把lower设为0进入下一轮。

 1 class Solution {
 2     public boolean checkValidString(String s) {
 3         int upper = 0;
 4         int lower = 0;
 5 
 6         for (char letter : s.toCharArray()) {
 7             switch (letter) {
 8                 case '(': lower++; upper++; break;
 9                 case ')': lower--; upper--; break;
10                 case '*': lower--; upper++; break;
11             }
12             if (upper < 0) return false;
13             if (lower < 0) lower = 0;
14         }
15         return lower == 0;
16     }
17 }