Longest Palindromic Substring

class Solution:
    def longestPalindrome(self, s: str) -> str:
        res=[None,None]
        n=len(s)
        global ans
        ans=0
        def find(i,j):
            global ans
            l,r=i,j
            while l>=0 and r<n:
                if s[l]==s[r]:
                    if r-l+1>ans:
                        ans=r-l+1
                        res[0]=l;res[1]=r
                    l,r=l-1,r+1
                else:
                    break

        for i in range(n):
            find(i,i)
            if i<n-1:
                find(i,i+1)
        l,r=res
        return s[l:r+1]