https://youtu.be/IWvbPIYQPFM?t=537

ContiguousSearch.php

<?php
/**
 * Author: git@ashbeats
 * Date: 12/2/2018
 * Time: 2:45 AM
 */


/**
 * Late night code after stumbling on https://youtu.be/IWvbPIYQPFM?t=537 and a ton of beers.
 *
 * Since the questioner was vague on the i/o, I'll assume it's a 2D Vector representing image pixels.
 *
 * I need to find the largest group of contiguous pixels. [ one or more sides touch a a pixel of the same color. ]
 *
 * <usage>
 * $input[ 0 ][ 0 ] = 'a';
 * $input[ 0 ][ 1 ] = 'b';
 * $input[ 0 ][ 2 ] = 'b';
 *
 * $input[ 1 ][ 0 ] = 'c';
 * $input[ 1 ][ 1 ] = 'c';
 * $input[ 1 ][ 2 ] = 'b';
 *
 * $input[ 2 ][ 0 ] = 'c';
 * $input[ 2 ][ 1 ] = 'a';
 * $input[ 2 ][ 2 ] = 'c';
 *
 * $input[ 3 ][ 0 ] = 'c';
 * $input[ 3 ][ 1 ] = 'c';
 * $input[ 3 ][ 2 ] = 'a';
 *
 *
 * $search = new ContiguousSearch( $input );
 * foreach ( $search->find() as $value => $count ) {
 * echo "\nPixel Value: '$value' has $count contiguous sides";
 * }
 *
 * </usage>
 *
 */
class ContiguousSearch
{
    // note: using array keys as a mock hashtable as keys are O(1) in php
    var $input2d = [];
    var $width = [];
    var $height = [];

    function __construct($input2d)
    {
        $this->input2d = $input2d;
        $this->width = count( $this->input2d );
        $this->height = count( current( $this->input2d ) );
    }

    function get($x, $y)
    {
        return $this->input2d[ $x ][ $y ] ?? null;
    }

    function compareThyNeighbours($x, $y)
    {
        $referencePoint = $this->get( $x, $y );
        $sides = [
            $referencePoint === $this->get( $x + 1, $y ),
            $referencePoint === $this->get( $x - 1, $y ),
            $referencePoint === $this->get( $x, $y + 1 ),
            $referencePoint === $this->get( $x, $y - 1 ),
        ];

        return [
            count( array_filter( $sides ) ), // todo - get rid of array filter but not much optimization headroom as it will still be O(4)
            $referencePoint,
        ];
    }

    /**
     * Find all contiguous pixels.
     *
     * @return array
     */
    function find()
    {
        $response = [];
        for ( $x = 0; $x < $this->width; $x++ ) {
            for ( $y = 0; $y < $this->height; $y++ ) {
                list( $match, $value ) = $this->compareThyNeighbours( $x, $y );
                if ( $match > 0 ) {
                    $response[ $value ][] = $match;
                }
            }
        }

        $matches = [];
        foreach ( array_keys( $response ) as $value ) {
            $matches[ $value ] = count( $response[ $value ] );
        }
        return $matches;
    }
}


# prep data
$input = [];
$input[ 0 ][ 0 ] = 'a';
$input[ 0 ][ 1 ] = 'b';
$input[ 0 ][ 2 ] = 'b';

$input[ 1 ][ 0 ] = 'c';
$input[ 1 ][ 1 ] = 'c';
$input[ 1 ][ 2 ] = 'b';

$input[ 2 ][ 0 ] = 'c';
$input[ 2 ][ 1 ] = 'a';
$input[ 2 ][ 2 ] = 'c';

$input[ 3 ][ 0 ] = 'c';
$input[ 3 ][ 1 ] = 'c';
$input[ 3 ][ 2 ] = 'a';

# execute
$search = new ContiguousSearch( $input );
foreach ( $search->find() as $value => $count ) {
    echo "\n'$value' occupies at max, $count contiguous pixels";
}