CheckIO The Longest Palindromic

Write a function that finds the longest palindromic substring of a given string. Try to be as efficient as possible!
If you find more than one substring you should return the one which is closer to the beginning.
Input: A text as a string.
Output: The longest palindromic substring.
Precondition: 1 < |text| ≤ 20
The text contains only ASCII characters.

解法一：

# 判断是否回文def isPalindrome(s, start, end):        while start < end:            if s[start] != s[end]:                return False            start += 1            end -= 1        return Truedef longest_palindromic(s):        max, left, right = 0, 0, 0        for i in range(len(s)):            j = i+1            while j < len(s):                if isPalindrome(s, i, j):                    if (j-i+1) > max:                        left, right = i, j                        max = j - i + 1                j += 1        print (left, right, max)        return s[left:right+1]

循环穷举，范围从小到大。

解法二：

# 0:9 0:8 1:9 ....def longest_palindromic(text):    s = len(text)    for size in range(s)[::-1]:        for index in range(s - size):            word = text[index:index + size + 1]            if word == word[::-1]:                return word
同样穷举，范围从大到小。

解法三:

from itertools import combinations as Cdef longest_palindromic(text):    subs = (text[start: end] for start, end in C(range(len(text) + 1), 2))    print(list(C(range(len(text) + 1), 2)))    return max((s for s in subs if s == s[::-1]), key=len)
其中的subs = (text[start: end] for start, end in C(range(len(text) + 1), 2))语句，直接
穷举出text字符串所有的子串，此方法很python。