<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)
        f = [[False for j in range(n)] for i in range(n)]
        for i in range(n):
            f[i][i] = True
        ans = s[0]
        for i in range(n - 1):
            f[i][i + 1] = s[i] == s[i + 1]
            if s[i] == s[i + 1]:
                ans = s[i: i + 2]
        for l in range(3, len(s) + 1):
            for i in range(n):
                j = i + l - 1
                if j >= n:
                    break
                f[i][j] = s[i] == s[j] and f[i + 1][j - 1]
                if f[i][j]:
                    ans = s[i: j + 1]
        return ans
        
25100