TLE for recursion.

Inspired by https://discuss.leetcode.com/topic/103946/o-n-java-solution

We always wants the two condition to be True, so scan the string

I write it in python with some modifications

```
class Solution(object):
def checkValidString(self, s):
"""
:type s: str
:rtype: bool
"""
leftcnt, rightcnt = 0, 0
for i in range(len(s)):
if s[i] == ')':
leftcnt -= 1
else:
leftcnt += 1
if leftcnt < 0:
return False
if s[len(s)-i-1] == '(':
rightcnt -= 1
else:
rightcnt += 1
if rightcnt < 0:
return False
return True
```