<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 {
private:
    string expandUntilPalindrome(string s, string max_str, int l, int r) {
      while (l >= 0 && r < s.size() && s[l] == s[r]) {
        if((r - l + 1) > max_str.size()) {
          max_str = s.substr(l, r - l + 1);
        }
        l -= 1;
        r += 1;
      }
      return max_str;
    }

public:
    string longestPalindrome(string s) {
      string str = "";
      for (int i = 0; i < s.size(); i++) {
        str = expandUntilPalindrome(s, str, i, i);
        str = expandUntilPalindrome(s, str, i, i + 1);
        cout << i << ": " << str << "\n";
      }
      return str;
    }
};
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