查找字符串最长回文

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:

Input: "babad"
Output: "bab"

• Note: “aba” is also a valid answer.

• Example:

Input: "cbbd"
Output: "bb"

思路

1. 回文有奇回文和偶回文，abcba是奇回文，abccba是偶回文

2. 回文都是中心对称，找到对称点后，同时向前后寻找回文的最长串即可

3. 奇回文和偶回文可以归为同一种情况，即abcba以c为对称点，abccba以cc为对称点，但为了代码可读性，可以分开讨论

代码

class Solution(object):

    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        self.maxlen = 0
        self.retstr = ''
        if len(s) < 2:
            return s
        for i in range(len(s)):
            self.__find_palindrome(s, i, i) #奇回文
            self.__find_palindrome(s, i, i+1) #偶回文
        return self.retstr


    def __find_palindrome(self, s, j, k):
        while j >= 0 and k < len(s) and s[j] == s[k]:
            j -= 1
            k += 1
        if self.maxlen < k - j + 1:
            self.maxlen = k - j + 1
            self.retstr = s[j+1:k]

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