5. Longest Palindromic Substring

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

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

Example 2:

Input: "cbbd"
Output: "bb"

class Solution(object):
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        
        res = ""
        
        # 遍历字符串，以单字符或双字符为中心，向两边扩展，保留最大长度
        for i in xrange(len(s)):
            odd = self.helper(s, i, i)
            even = self.helper(s, i, i+1)
            
            res = max(res, odd, even, key=len)
        
        return res
        
    def helper(self, s, l, r):
        
        while l >= 0 and r < len(s) and s[l] == s[r]:
            l -= 1
            r += 1
        
        return s[l+1:r]