gistfile1.php

<?php


//  dnepr -> ujgorod -> poltava -> kharkov -> kiev => odessa -> lvov -> doceck
$tickets = [
	['start' => 'kiev', 'finish' => 'odessa'],
	['start' => 'dnepr', 'finish' => 'ujgorod'],
	['start' => 'odessa', 'finish' => 'lvov'],
	['start' => 'kharkov', 'finish' => 'kiev'],
	['start' => 'poltava', 'finish' => 'kharkov'],
	['start' => 'ujgorod', 'finish' => 'poltava'],
	['start' => 'lvov', 'finish' => 'doneck'],
	['start' => 'lvov1', 'finish' => 'doneck'],
	['start' => 'lvov', 'finish' => 'doneck1']
];


function getStacks($list) {

	$stacks = [];

	foreach ($list as $item) {

		$stacked = false;

		// crunch
		if (!isset($item['items'])) {
			$item['items'] = [$item];
		}

		foreach ($stacks as &$stack) {

			if ($stack['start'] == $item['finish'] && !$stacked) {
				$stack['items'] = array_merge($item['items'], $stack['items']); // unshift
				$stack['start'] = $item['start'];
				$stacked = true;
			}

			if ($stack['finish'] == $item['start'] && !$stacked) {
				$stack['items'] = array_merge($stack['items'], $item['items']); // push
				$stack['finish'] = $item['finish'];
				$stacked = true;
			}
		}

		if ($stacked == false ) {
			$stacks[] = $item;
		}

	}

	return $stacks;

}

$iterations = 0;

$initialTicketsSize = sizeof($tickets);

do {
	$tickets = getStacks($tickets);
	$iterations++;
} while (sizeof($tickets) != 1 && $iterations <= $initialTicketsSize);


var_dump($tickets);
var_dump($iterations);