Longest Palindromic Substring

classLPS.php

<?php
/**
 * Problem: https://leetcode.com/problems/longest-palindromic-substring/
 *
 * Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
 *
 * Summary: This article is for intermediate readers. It introduces the following ideas: Palindrome, Dynamic Programming and String Manipulation. Make sure you understand what a palindrome means. A palindrome is a string which reads the same in both directions. For example, SS = "aba" is a palindrome, SS = "abc" is not.
 * 
 * Letcode Submission: https://leetcode.com/submissions/detail/355341568/
 */

/**
 * Class Solution
 */
class Solution {

	/**
	 * @param String $data Input String.
	 * @return false|string|null $longest_str Longest String.
	 */
	function longestPalindrome($data) {

		if(empty($data))
		{
			return "";
		}

		$rev = strrev($data);
		$len = strlen($data);
		$longest_len = 0;
		$longest_str = null;

		for ($i = 0; $i < $len; ++$i)
		{
			for ($j = $len - $i; $j > $longest_len; --$j)
			{
				if (substr($data, $i, $j) == substr($rev, $len-$i-$j, $j))
				{
					$longest_len = $j;
					$longest_str = substr($data, $i, $j);
					break;
				}
			}
		}
		return $longest_str;
	}
}