<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:
    def longestPalindrome(self, s: str) -> str:
        n = len(s)
        if n <= 1:
            return s
        p = [[False]*(n+1-i) for i in range(n)]
        for i in range(n):
            p[i][0] = True
            p[i][1] = True
        maxlength = 1
        for length in range(2,n+1):
            for i in range(n-length+1):
                if s[i] == s[i+length-1] and p[i+1][length-2]:
                    p[i][length] = True
                    maxlength = length
        for i in range(n):
            if p[i][maxlength]:
                return s[i:i+maxlength]
20500