<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:
        if not s:
            return ""

        n = len(s)
        max_start = -1
        max_len = 0
        dp = [[False] * n for _ in range(n)]

        for i in range(n):
            dp[i][i] = True

        for j in range(0, n):
            for i in range(j, -1, - 1):
                if s[i] == s[j] and (j - i + 1 <= 2 or dp[i + 1][j - 1]):
                    dp[i][j] = True
                if dp[i][j] and j - i + 1 > max_len:
                    max_len = j - i + 1
                    max_start = i
        
        return "" if max_start == -1 else s[max_start:max_start + max_len]

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