Longest Palindromic Substring
<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>
Two Pointers; String; Dynamic Programming
class Solution {
    /**
     * @param string $s
     * @return string
     */
    function longestPalindrome($s) {
        $start = 0;
        $maxLength = 0;

        for ($i = 0; $i < strlen($s); $i++) {
            $len1 = $this->expandFromCenter($s, $i, $i);
            $len2 = $this->expandFromCenter($s, $i, $i + 1);

            $len = max($len1, $len2);

            if ($len > $maxLength) {
                $start = $i - intval(($len - 1) / 2);
                $maxLength = $len;
            }
        }

        return substr($s, $start, $maxLength);
    }

    function expandFromCenter($s, $left, $right) {
        while ($left >= 0 && $right < strlen($s) && $s[$left] === $s[$right]) {
            $left--;
            $right++;
        }

        return $right - $left - 1;
    }
}
