最长回文子串

题目描述：求一个字符串的最长回文子串

def maxhuiwen(s):
    k=len(s)
    martix=[[0 for i in range(k)] for j in range(k)]
    len1=0
    substr=""
    for j in range(k):
        for i in range(0,j+1):
            if j-i<=1:
                if s[i]==s[j]:
                    martix[i][j]=1
                    if len1<j-i+1:
                        len1=j-i+1
                        substr=s[i:j+1]
            else:
                if s[i] == s[j] and martix[i+1][j-1]:
                    martix[i][j] = 1
                    if len1 < j - i + 1:
                        len1 = j - i + 1
                        substr = s[i:j + 1]
    return substr