/**
  * @param Graph $graph
  * @param int $vertex
  */
 private function depthSearch(Graph $graph, int $vertex)
 {
     $this->marked[$vertex] = true;
     $this->count++;
     foreach ($graph->adjacent($vertex) as $adjacent) {
         if ($this->isMarked($adjacent)) {
             continue;
         }
         $this->depthSearch($graph, $adjacent);
     }
 }