题目

Given a string s, return the longest palindromic substring in s.

Example 1:

Input: s = "babad"
Output: "bab"
Explanation: "aba" is also a valid answer.

Example 2:

Input: s = "cbbd"
Output: "bb"

Constraints:

• 1 <= s.length <= 1000
• s consist of only digits and English letters.

思路

遍历序列，求出所有可能的最大回文串，随后返回最大的一个。

代码

python版本：

class Solution:
    def longestPalindrome(self, s: str) -> str:
        res = ''
        maxn = 0

        def maxPalindromeLen(i):
            nonlocal res, maxn
            n = 1
            while i-n >= 0 and i+n < len(s) and s[i-n] == s[i+n]:
                n += 1
            if 2*n-1 > maxn:
                maxn = 2*n-1
                res = s[i-n+1:i+n]
            n = 0
            while i-n >= 0 and i+n+1 < len(s) and s[i-n] == s[i+1+n]:
                n += 1
            if 2*n > maxn:
                maxn = 2*n
                res = s[i-n+1:i+n+1]
        [maxPalindromeLen(i) for i in range(len(s))]
        return res