广度优先搜索与深度优先搜索

广度优先搜索的方式模仿二叉树的层序遍历，利用一个队列存储adjacency vertex,利用white,gray and black 标示节点的三种状态。white表示未访问，gray表示已发现但为访问，也就是处于队列节点的状态，black表示已访问，也就是从队列中弹出的节点。

/*图算法*/

	//breath first search

	public static<T> LinkedList<T> bfs(DiGraph<T> g,T sVertex)

	{

		//queue stores adjaceent vertices; list stores visited vertices

		LinkedQueue<T> visitQueue=new LinkedQueue();

		LinkedList<T> visitList=new LinkedList<T>();

		Set<T> edgeSet;

		Iterator<T> edgeIter;

		T currVertex=null,neighborVertex=null;

		//check that starting vertex is valid

		if(!g.containsVertex(sVertex))

		{

			throw new IllegalArgumentException("bfs():starting vertex not in the graph");

		}

		//color all vertices WHITE

		g.colorWhite();

		

		//initialize queue with starting vertex

		visitQueue.push(sVertex);

		while(!visitQueue.isEmpty())

		{

			

			//remove a vertex from the queue,color it blace and add to the list of visited vertices

			currVertex=visitQueue.pop();

			g.setColor(currVertex, VertexColor.BLACK);

			//obtain the set of neighbors for current vertex

			visitList.add(currVertex);

			edgeSet=g.getNeighbors(currVertex);

			//sequence through the neighbors and look for vertices

			//that have not been visited

			edgeIter=edgeSet.iterator();

			while(edgeIter.hasNext())

			{

				neighborVertex=edgeIter.next();

				if(g.getColor(neighborVertex)==VertexColor.WHITE)

				{

					//color unvisited vertex Gray and push it onto queue

					g.setColor(neighborVertex, VertexColor.GRAY);

					visitQueue.push(neighborVertex);

				}

			}

			

		}

		return visitList;

	}

	/*图算法*/

广度优先访问的是当前节点可达的所有节点

 

深度优先，模仿二叉树的后序遍历，只有在访问所有或许节点后方可访问。深度优先是一种可以区分节点发现时间和结束时间的算法，并且可以利用深度优先发现图中环的存在。深度优先返回的列表是按照每个节点结束时间相反的顺序。

	public static<T> void dfsVisit(DiGraph<T> g,T sVertex,LinkedList<T>dfsList,boolean checkForCycle)

	{

		T neighborVertex;

		Set<T> edgeSet;

		//iterator to scan the adjacency set of a vertex

		Iterator<T> edgeIter;

		VertexColor color;

		if(!g.containsVertex(sVertex))

		throw  new IllegalArgumentException("dfs():vertex not in the graph");

		//color vertex gray note its discovery

		g.setColor(sVertex,VertexColor.GRAY);

		edgeSet=g.getNeighbors(sVertex);

		//sequence through the adjacency set and look for vertices that are not yet discovered

		//recursively call dfsVisit() for each such vertex if a vertex in the adjacency list is GRAY, the vertex was discovered during a previous call and there

		//is a cycle that begins and ends at the vertex;if checkForCycle is true ,throw an Exception

		edgeIter=edgeSet.iterator();

		while(edgeIter.hasNext())

		{

			neighborVertex=edgeIter.next();

			color=g.getColor(neighborVertex);

			if(color==VertexColor.WHITE)

				dfsVisit(g,neighborVertex,dfsList,checkForCycle);

			else if(color==VertexColor.GRAY&&checkForCycle)

			{

				throw new IllegalPathStateException("dfsVisit():graph has a cycle!");

			}

		}

		//finished with vertex sVertex; make it BLACK

		//and add it the front of dfsList

		g.setColor(sVertex,VertexColor.BLACK);

		dfsList.addFirst(sVertex);

		  

	}