leetcode 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"

分析：

遍历数组，判断以每个字符为中心分别向两头扩散，是否为回文序列。根据回文的长度分两种情况：偶数回文和奇数回文。

 def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        res = ''
        for i in range(len(s)):
            # odd case, like 'aba'
            tmp = self.helper(s, i, i)
            if len(tmp) > len(res):
                res = tmp
            # even case, like 'abba'
            tmp = self.helper(s, i, i+1)
            if len(tmp) > len(res):
                res = tmp
        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]