最长回文子串

题目

给定一个字符串s，找到s中最长的回文子串。你可以假设s的最大长度为1000。

思路

中心扩散
以str[i]为中心
或者如果str[i] == str[i + 1]，以str[i]和str[i + 1]这个整体为中心

class Solution:
    def longestPalindrome(self, s: str) -> str:
        length = len(s)
        if length <= 1: return s
        max_palindrome = ''
        for i in range(length):
            tmp = self.helper(s, i, length)
            max_palindrome = tmp if len(tmp) > len(max_palindrome) else max_palindrome
        return max_palindrome
    def helper(self, s, mid, length):
        # 以当前mid为中心遍历一遍
        left, right = mid - 1, mid + 1
        odd_str = s[mid]
        while left >= 0 and right < length and s[left] == s[right]:
            odd_str = s[left : right + 1]
            left -= 1
            right += 1
        even_str = s[mid]
        if mid + 1 < length and s[mid] == s[mid + 1]:
            left = mid
            right = mid + 1
            even_str = s[left : right + 1]
            while left >= 0 and right < length and s[left] == s[right]:
                even_str = s[left : right + 1]
                left -= 1
                right += 1
        return odd_str if len(odd_str) > len(even_str) else even_str

https://leetcode-cn.com/problems/longest-palindromic-substring/solution/zui-chang-hui-wen-zi-chuan-by-leetcode-solution/

• 需要进一步完善