枚举中心点

class Solution:
    def longestPalindrome(self, s: str) -> str:
        res = ''
        i = 0
        while i < len(s):
            j = i; k = i # 奇数情况，j,k为左右指针，起点相同
            while j >= 0 and k < len(s) and s[j] == s[k]:
                if len(res) < k - j + 1:
                    res = s[j : k + 1]
                j -= 1
                k += 1

            j = i; k = i + 1 # 偶数情况，j,k为左右指针，起点不同
            while j >= 0 and k < len(s) and s[j] == s[k]:
                if len(res) < k - j + 1:
                    res = s[j : k + 1]
                j -= 1
                k += 1

            i += 1

        return res