<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 iter_s(self, i, s, ans, dulip=False):
        if i < 0 or i >= len(s):
            return
        l, r = i - 1, i + 1
        
        if dulip and r < len(s):
            if s[i] != s[r]:
                return
            r += 1

        ans.add(s[i:r])
        
        if l < 0 or r >= len(s):
            return
        while l >= 0 and r < len(s):
            if s[l] != s[r]:
                break
            l -= 1
            r += 1
        ans.add(s[l + 1: r])
    
    def longestPalindrome(self, s: str) -> str:
        if len(s) == 1:
            return s
        ans = set()
        for i, c in enumerate(s):
            self.iter_s(i, s, ans)
            self.iter_s(i, s, ans, dulip=True)
        return max(ans, key=lambda x: len(x))
        
17100