<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 validatePalindrome(self, s: str) -> bool:
        for i in range(0, len(s)//2):
            if(s[i] != s[-i-1]):
                return False
        return True

    def longestPalindrome(self, s: str) -> str:
        if(len(s) == 2):
            if(s[0] == s[1]):
                return s
            else:
                return s[0]
        if(len(s) <= 1):
            return s

        s2 = s+"$"
        palindorme_cands = []
        for shift in range(0, len(s2)):
            common_letters=""
            for i in range(0, len(s)):
                if(s[i] == s2[len(s) - (i+shift)%len(s2) - 1]):
                    common_letters += s[i]
                else:
                    if(len(common_letters) > 1):
                        palindorme_cands.append(common_letters)
                    common_letters = ""
            if(len(common_letters) > 1):
                palindorme_cands.append(common_letters)

        longest_palindrome = s[0]
        for cand in palindorme_cands:
            if(Solution().validatePalindrome(cand) and len(cand) > len(longest_palindrome)):
                longest_palindrome = cand


        return longest_palindrome
18500