BFS - Breadth First Search

The below is sample code to iterate through graph nodes using breadth first search -

			
//build graph nodes
var tree = new Dictionary<int, List<int>>();
tree.Add(1, new List<int> { 2, 3, 4 });
tree.Add(2, new List<int> { 5 });
tree.Add(3, new List<int> { 6, 7});
tree.Add(4, new List<int> { 8 });
tree.Add(5, new List<int> { 9 });
tree.Add(6, new List<int> { 10 });

//get root element of graph
Queue q = new Queue();
q.Enqueue(tree.ElementAt(0).Key);

HashSet<int> visitiedNode = new HashSet<int>();

//iterate queue to visit nodes
while(q.Count > 0)
{
	//dequeue 1st element
	var item = Convert.ToInt32(q.Dequeue());

	//check if element already visited
	if (visitiedNode.Contains(item))
		continue;

	//add element to visited list
	visitiedNode.Add(item);

	Console.WriteLine(item);

	//get next adjecent nodes
	tree.TryGetValue(item, out List<int> adjcentNodes);

	//enqueue new nodes to the queue
	if (adjcentNodes != null)
		adjcentNodes.ForEach(a => q.Enqueue(a));
}

This code iterates through each node and can be modified to check on given key.