深拷贝二叉树

java语言递归复制二叉树，mark一下

public TreeNode copyTreeNode(TreeNode root) {
        TreeNode copyTree = null;
        if (root != null) {
            copyTree = new TreeNode (root.val);
            copyTree.left = copyTreeNode(root.left);
            copyTree.right = copyTreeNode(root.right);
        }
        return copyTree;
}