编码挑战：通过解决问题参与和学习

input: s = "babad"output: "bab"note: "aba" is also a valid answer.

def longest_palindrome_brute_force(s):    def is_palindrome(sub):        return sub == sub[::-1]    n = len(s)    longest = ""    for i in range(n):        for j in range(i, n):            if is_palindrome(s[i:j+1]) and len(s[i:j+1]) > len(longest):                longest = s[i:j+1]    return longestprint(longest_palindrome_brute_force("babad"))  # output: "bab" or "aba"

def longest_palindrome_expand_center(s):    def expand_around_center(left, right):        while left >= 0 and right < len(s) and s[left] == s[right]:            left -= 1            right += 1        return s[left+1:right]    longest = ""    for i in range(len(s)):        # odd length palindromes        odd_palindrome = expand_around_center(i, i)        if len(odd_palindrome) > len(longest):            longest = odd_palindrome        # even length palindromes        even_palindrome = expand_around_center(i, i+1)        if len(even_palindrome) > len(longest):            longest = even_palindrome    return longestprint(longest_palindrome_expand_center("babad"))  # output: "bab" or "aba"

def longest_palindrome_dp(s):    n = len(s)    if n == 0:        return ""    dp = [[False] * n for _ in range(n)]    start, max_length = 0, 1    for i in range(n):        dp[i][i] = True    for length in range(2, n+1):        for i in range(n-length+1):            j = i + length - 1            if s[i] == s[j]:                if length == 2 or dp[i+1][j-1]:                    dp[i][j] = True                    if length > max_length:                        start = i                        max_length = length    return s[start:start+max_length]print(longest_palindrome_dp("babad"))  # Output: "bab" or "aba"