Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.
Example 2:

Input: "cbbd"
Output: "bb"

class Solution:
    def longestPalindrome(self,s):
        str_length = len(s)
        max_length = 0
        start = 0
        for i in range(str_length):
            if i - max_length >= 1 and s[i - max_length - 1:i + 2] == s[i - max_length - 1:i + 2][::-1]:
                start = i - max_length - 1
                max_length += 2
                continue
            if i - max_length >= 0 and s[i - max_length:i + 2] == s[i - max_length:i + 2][::-1]:
                start = i - max_length
                max_length += 1
        return s[start:start + max_length+1]