最长回文子串

class Solution:
    def longestPalindrome(self, s: str) -> str:
        if not s: return 0
        t = "?#"
        for c in s:
            t += c + '#'
        t += '!'
        n = len(t)
        f = [1 for _ in range(n)]
        res, im , rm = '', 0, 0
        for i in range(2, n - 2):
            f[i] = min(rm - i + 1, f[2 * im - i]) if i < rm else 1
            # print(f)
            while t[i + f[i]] == t[i - f[i]]:
                f[i] += 1
            if i + f[i] - 1 > rm:
                rm = i + f[i] - 1
                im = i 
            if 2 * f[i] - 1 > len(res):
                res = t[i - f[i] + 1: i + f[i]]
        return res[1::2]
        

添加#是解决奇偶位统一处理