you do not need to verify the s is odd length or even length

```
class Solution(object):
def longestPalindrome(self, s):
"""
:type s: str
:rtype: str
"""
mx, n, res = 0,len(s), []
for i in range(n*2 - 1):
left = i/2
right = left + i%2
while left >= 0 and right < n and s[left] == s[right]:
left -= 1
right += 1
if mx < right - left - 1:
res = s[left + 1:right]
mx = right - left - 1
return res
```