二叉树的遍历 Java版

 下面给出二叉树的前序、后序、中序遍历的递归和非递归加上层序的非递归算法

/*前序遍历，递归*/
	public void preorder(TreeNode root){
		TreeNode node = root;
		if(node!=null){
			visit(node);
			preorder(node.left);
			preorder(node.right);
		}
	}
	/*中序遍历，递归*/
	public void inorder(TreeNode root){
		TreeNode node = root;
		if(node!=null){
			inorder(node.left);
			visit(node);	
			inorder(node.right);
		}
	}
	/*后序遍历，递归*/
	public void postorder(TreeNode root){
		TreeNode node = root;
		if(node!=null){
			postorder(node.left);
			postorder(node.right);
			visit(node);
		}
	}
	/*前序遍历非递归*/
	public void preorderNonRecursion(TreeNode root){
		if(root==null)
			return ;
		TreeNode node = root;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		while(node!=null||!stack.isEmpty()){
			while(node!=null){
				visit(node);
				stack.push(node);
				node = node.left;	
			}
			TreeNode tn = stack.pop();
			node = tn.right;
		}
	}
	/*中序遍历非递归*/
	public void inorderNonRecursion(TreeNode root){
		if(root==null)
			return ;
		TreeNode node = root;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		while(node!=null||!stack.isEmpty()){
			while(node!=null){
				stack.push(node);
				node = node.left;	
			}
			TreeNode tn = stack.pop();
			visit(tn);
			node = tn.right;
		}
	}
	/*后序遍历非递归*/
	public void postorderNonRecursion(TreeNode root){
		if(root==null)
			return ;
		TreeNode node = root;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		TreeNode previst = null;//记录上一次访问的节点
		while(node!=null||!stack.isEmpty()){
			while(node!=null){
				stack.push(node);
				node = node.left;	
			}
			TreeNode tn = stack.peek();
			//如果该节点的右孩子为空，或者改节点的右孩子已经被访问过，那么访问该节点
			if(tn.right==null||tn.right==previst){
				visit(tn);
				stack.pop();
				previst = tn;
			}else
				node = tn.right;
		}
	}
	/*层序遍历，非递归*/
	public void cengNonRecursion(TreeNode root){
		if(root==null)
			return ;
		TreeNode node = root;
		Queue<TreeNode> queue = new LinkedList<TreeNode>();
		queue.add(node);
		while(!queue.isEmpty()){
			TreeNode tn = queue.poll();
			visit(tn);
			if(tn.left!=null)
				queue.add(tn.left);
			if(tn.right!=null)
				queue.add(tn.right);
		}
		
	}
	public void visit(TreeNode node){
		System.out.print(node.val+" ");
	}