递归二叉树的序列打印

请用递归方式实现二叉树的先序、中序和后序的遍历打印。

给定一个二叉树的根结点root，请依次返回二叉树的先序，中序和后续遍历(二维数组的形式)。

import java.util.*;/*public class TreeNode {    int val = 0;    TreeNode left = null;    TreeNode right = null;    public TreeNode(int val) {        this.val = val;    }}*/public class TreeToSequence {public int[][] convert(TreeNode root) {List<Integer> first = new LinkedList<Integer>();List<Integer> middle = new LinkedList<Integer>();List<Integer> back = new LinkedList<Integer>();getFirst(first, root);getMiddle(middle, root);getBack(back, root);int[][] result = new int[3][first.size()];for (int i = 0; i < first.size(); i++) {result[0][i] = first.get(i);result[1][i] = middle.get(i);result[2][i] = back.get(i);} return result;}public void getFirst(List<Integer> first, TreeNode root) {if (root == null)return;first.add(root.val);getFirst(first, root.left);getFirst(first, root.right);}public void getMiddle(List<Integer> first, TreeNode root) {if (root == null)return;getMiddle(first, root.left);first.add(root.val);getMiddle(first, root.right);}public void getBack(List<Integer> first, TreeNode root) {if (root == null)return;getBack(first, root.left);getBack(first, root.right);first.add(root.val);}}