<p>Given a string <code>s</code>, return <em>the longest</em> <span data-keyword="palindromic-string"><em>palindromic</em></span> <span data-keyword="substring-nonempty"><em>substring</em></span> in <code>s</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;babad&quot;
<strong>Output:</strong> &quot;bab&quot;
<strong>Explanation:</strong> &quot;aba&quot; is also a valid answer.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;cbbd&quot;
<strong>Output:</strong> &quot;bb&quot;
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 1000</code></li>
	<li><code>s</code> consist of only digits and English letters.</li>
</ul>

class Solution(object):

    def is_palindrome(self, s: str) -> bool:
        for i in range(len(s) // 2):
            if s[i] != s[len(s) - i - 1]:
                return False
        return True

    def longestPalindrome(self, s: str, p: str = "") -> str:
        for i in range(max(0, len(s) - len(p))):
            a = s[: len(s) - i]
            if self.is_palindrome(a):
                p = a
                break
        alternative_p = ""
        if len(p) < len(s) - 1:
            alternative_p = self.longestPalindrome(s[1:], p)
        return p if len(p) > len(alternative_p) else alternative_p
17500