//节点
    public static class Node {
        public int value;
        public Node left;
        public Node right;

        public Node(int data) {
            this.value = data;
        }
    }

//递归遍历
//先序遍历
    public static void preOrderRecur(Node head) {
        if (head == null) {
            return;
        }
        System.out.print(head.value + " ");
        preOrderRecur(head.left);
        preOrderRecur(head.right);
    }

//中序遍历
    public static void inOrderRecur(Node head) {
        if (head == null) {
            return;
        }
        inOrderRecur(head.left);
        System.out.print(head.value + " ");
        inOrderRecur(head.right);
    }
//后序遍历
    public static void posOrderRecur(Node head) {
        if (head == null) {
            return;
        }
        posOrderRecur(head.left);
        posOrderRecur(head.right);
        System.out.print(head.value + " ");
    }

//非递归遍历
//先序
    public static void preOrder(Node head) {
        System.out.print("pre-order: ");
        if (head != null) {
            Stack<Node> stack = new Stack<Node>();
            stack.add(head);
            while (!stack.isEmpty()) {
                head = stack.pop();
                System.out.print(head.value + " ");
                if (head.right != null) {
                    stack.push(head.right);
                }
                if (head.left != null) {
                    stack.push(head.left);
                }
            }
        }
        System.out.println();
    }


//中序
    public static void inOrder(Node head) {
        System.out.print("in-order: ");
        if (head != null) {
            Stack<Node> stack = new Stack<Node>();
            while (!stack.isEmpty() || head != null) {
                if (head != null) {
                    stack.push(head);
                    head = head.left;
                } else {
                    head = stack.pop();
                    System.out.print(head.value + " ");
                    head = head.right;
                }
            }
        }
        System.out.println();
    }

//后序遍历
    public static void posOrder(Node head) {
        System.out.print("pos-order: ");
        if (head != null) {
            Stack<Node> s1 = new Stack<Node>();
            Stack<Node> s2 = new Stack<Node>();
            s1.push(head);
            while (!s1.isEmpty()) {
                head = s1.pop();
                s2.push(head);
                if (head.left != null) {
                    s1.push(head.left);
                }
                if (head.right != null) {
                    s1.push(head.right);
                }
            }
            while (!s2.isEmpty()) {
                System.out.print(s2.pop().value + " ");
            }
        }
        System.out.println();
    }
//层次遍历
    public static void Level(Node head) {
        if (head == null) {
            return;
        }
        Queue<Node> queue = new LinkedList<Node>();
        int level = 1;
        Node last = head;
        Node nLast = null;
        queue.offer(head);
        System.out.print("Level " + (level++) + " : ");
        while (!queue.isEmpty()) {
            head = queue.poll();
            System.out.print(head.value + " ");
            if (head.left != null) {
                queue.offer(head.left);
                nLast = head.left;
            }
            if (head.right != null) {
                queue.offer(head.right);
                nLast = head.right;
            }
            if (head == last && !queue.isEmpty()) {
                System.out.print("\nLevel " + (level++) + " : ");
                last = nLast;
            }
        }
        System.out.println();
    }