1）逐个遍历，向两边扩散。分单一扩散（奇数），和双字符扩散（偶数）两种情况考虑

时间复杂度较高

class Solution:
    def longestPalindrome(self, s: str) -> str:
        length = len(s)
        maxlength = 0
        for i in range(length):
            l = i
            r = i
            while l >= 0 and r < length and s[l] == s[r]:
                if maxlength < len(s[l:r+1]):
                    maxlength = len(s[l:r+1])
                    result = s[l:r+1]
                l -= 1
                r += 1
            l = i
            r = i+1
            while l >= 0 and r < length and s[l] == s[r]:
                if maxlength < len(s[l:r+1]):
                    maxlength = len(s[l:r+1])
                    result = s[l:r+1]
                l -= 1
                r += 1
        return result