PHP Program for Longest Palindromic Subsequence | DP-12

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Given a sequence, find the length of the longest palindromic subsequence in it.

As another example, if the given sequence is “BBABCBCAB”, then the output should be 7 as “BABCBAB” is the longest palindromic subsequence in it. “BBBBB” and “BBCBB” are also palindromic subsequences of the given sequence, but not the longest ones.

1) Optimal Substructure:
Let X[0..n-1] be the input sequence of length n and L(0, n-1) be the length of the longest palindromic subsequence of X[0..n-1].
If last and first characters of X are same, then L(0, n-1) = L(1, n-2) + 2.
Else L(0, n-1) = MAX (L(1, n-1), L(0, n-2)).
Following is a general recursive solution with all cases handled.

Dynamic Programming Solution

PHP

<?php
// A Dynamic Programming based 
// PHP program for LPS problem
// Returns the length of the 
// longest palindromic 
// subsequence in seq
 
// A utility function to get
// max of two integers
// function max( $x, $y)
// { return ($x > $y)? $x : $y; }
 
// Returns the length of the
// longest palindromic 
// subsequence in seq
function lps($str)
{
$n = strlen($str);
$i; $j; $cl;
 
// Create a table to store
// results of subproblems
$L[][] = array(array()); 
 
 
// Strings of length 1 are
// palindrome of length 1
for ($i = 0; $i < $n; $i++)
    $L[$i][$i] = 1;
 
    // Build the table. Note that 
    // the lower diagonal values 
    // of table are useless and 
    // not filled in the process. 
    // The values are filled in a 
    // manner similar to Matrix 
    // Chain Multiplication DP 
    // solution (See 
    // https://www.zambiatek.com/matrix-chain-multiplication-dp-8/).
    // cl is length of substring
    for ($cl = 2; $cl <= $n; $cl++)
    {
        for ($i = 0; $i < $n - $cl + 1; $i++)
        {
            $j = $i + $cl - 1;
            if ($str[$i] == $str[$j] && 
                            $cl == 2)
            $L[$i][$j] = 2;
            else if ($str[$i] == $str[$j])
            $L[$i][$j] = $L[$i + 1][$j - 1] + 2;
            else
            $L[$i][$j] = max($L[$i][$j - 1], 
                             $L[$i + 1][$j]);
        }
    }
 
    return $L[0][$n - 1];
}
 
// Driver Code
$seq = 'GEEKS FOR GEEKS';
$n = strlen($seq);
echo "The length of the " . 
      "LPS is ", lps($seq);
 
// This code is contributed
// by shiv_bhakt.
?>