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/**
     * 前序遍历，递归实现
     */
    public void preOrder(TreeNode node){

        if(node != null){
            System.out.println(node.val);
            preOrder(node.left);
            preOrder(node.right);
        }
    }

    /**
     * 中序遍历，递归实现
     */
    public void inOrder(TreeNode node){

        if(node != null){
            inOrder(node.left);
            System.out.println(node.val);
            inOrder(node.right);
        }
    }

    /**
     * 后序遍历，递归实现
     */
    public void backOrder(TreeNode node){

        if(node != null){
            backOrder(node.left);
            backOrder(node.right);
            System.out.println(node.val);
        }
    }

    /**
     * 深度优先遍历（使用栈的先进后出特性，将右子树先压栈
     */
    public void depthOrder(){

        if(root == null){
            System.out.println("empty tree!");
            return;
        }

        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(stack.isEmpty() == false){
            TreeNode node = stack.pop();
            System.out.println(node.val + " ");
            if(node.right != null){
                stack.push(node.right);
            }
            if(node.left != null){
                stack.push(node.left);
            }
        }
        System.out.println();

    }

    /**
     * 广度优先遍历，使用队列实现
     */
    public void levelOrder(){

        if(root == null){
            System.out.println("empty tree");
            return;
        }
        ArrayDeque<TreeNode> queue = new ArrayDeque<TreeNode>();
        queue.add(root);
        while(queue.isEmpty() == false){
            TreeNode node = queue.remove();
            System.out.println(node.val + " ");
            if(node.left != null){
                queue.add(node.left);
            }
            if(node.right != null){
                queue.add(node.right);
            }
        }
        System.out.println();
    }