二叉树结点的定义与先序（中左右）、中序（左中右）、后序（左右中）遍历，顺便写个最大深度，都是递归实现，之后再学习非递归的方法。

 

先序遍历的结果为：5,2,4,1,3,8,4,2,9，中序遍历的结果：4,2,1,5,4,8,2,3,9，后序遍历结果：4,1,2,4,2,8,9,3,5。

package leetcode;class TreeNode{    TreeNode left;    TreeNode right;    int val;    TreeNode(int val)    {        this.val = val;    }    static int getDepth(TreeNode root)    {        if(root==null)        {            return 0;        }        int right = getDepth(root.right);        int left = getDepth(root.left);        return left>right?left+1:right+1;    }    static void scanNodes(TreeNode root)    {        if(root==null)        {            return;        }        //System.out.println(root.val); //先序遍历        scanNodes(root.left);        //System.out.println(root.val); //中序遍历        scanNodes(root.right);        System.out.println(root.val); //后序遍历    }    public static void main(String[] args)    {        TreeNode root = new TreeNode(5);        TreeNode left1 = new TreeNode(2);        TreeNode left2 = new TreeNode(1);        TreeNode left3 = new TreeNode(4);        TreeNode right1 = new TreeNode(3);        TreeNode right2 = new TreeNode(8);        TreeNode right3 = new TreeNode(9);        TreeNode right4 = new TreeNode(4);        TreeNode right5 = new TreeNode(2);        root.left = left1;        left1.right = left2;        left1.left = left3;        root.right = right1;        right1.left = right2;        right1.right = right3;        right2.left = right4;        right2.right = right5;        scanNodes(root);        System.out.println("The depth of BinaryTree is: "+getDepth(root));    }}