LeetCode重点题系列之【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"

First, let's traverse the s, then expand each element, there are two possible situations, odd string or even string。

class Solution(object):
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        self.res=""
        def helper(l,r):
            while 0<=l<len(s) and 0<=r<len(s) and s[l]==s[r]:
                tmp=s[l:r+1]
                if len(tmp)>len(self.res):
                    self.res=tmp[:]
                l-=1
                r+=1
        for i in range(len(s)):
            l=r=i
            helper(l,r)
            l=i
            r=i+1
            helper(l,r)
        return self.res