字符串中最长的Palindrome没有循环Python 3

Question

I'm trying to make a function that finds the longest palindrome in a string without using for loops. If there are palindromes of the same length it with produce the one that comes first alphabetically. examples: "" => "", "bcd" => "b", "acaba" => aba

I found this on overflow which is similar except it uses for loops and it finds the first palindrome.

def palindromes(text):
    results = []

for i in range(len(text)):
    for j in range(0, i):
        chunk = text[j:i + 1]

        if chunk == chunk[::-1]:
            results.append(chunk)

return text.index(max(results, key=len)), results

A method I was thinking of using is to check if each substring is equal to it's reverse substring[::-1]. But I don't know how to get each possible substring. I know with recursion I could remove the last or first position of the string but that wouldn't check the substrings in the middle.

Answer 1

With recursion you can simplify to either match itself, or check recursively beginning of string without last character or end of string without first character:

def palindromes(text):
    if text == text[::-1]:
        return len(text)
    return max(palindromes(text[:-1]), palindromes(text[1:]))

The complexity is quite bad - exponential, though.