题解 | #实现二叉树先序，中序和后序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类 the root of binary tree
     * @return int整型二维数组
     */
    public int[][] threeOrders (TreeNode root) {
        // 前序遍历  根节点-左-右
        int a[][] = {Arrays.stream(diguiLeft(root, new ArrayList())).mapToInt(Integer::valueOf).toArray(),Arrays.stream(diguiMiddle(root, new ArrayList())).mapToInt(Integer::valueOf).toArray(),
        Arrays.stream(diguiRight(root, new ArrayList())).mapToInt(Integer::valueOf).toArray()};
        // 中序遍历  左-根节点-右
        return a;
    }
    public static Integer[] diguiLeft(TreeNode root, ArrayList<Integer> arry) {
        if (root != null) {
            arry.add(root.val);
            diguiLeft(root.left, arry);
            diguiLeft(root.right, arry);
        }
        return arry.toArray(new Integer[arry.size()]);
    }
    public static Integer[] diguiMiddle (TreeNode root, ArrayList<Integer> arry) {
        if (root != null) {
            diguiMiddle(root.left, arry);
            arry.add(root.val);
            diguiMiddle(root.right, arry);
        }
        return arry.toArray(new Integer[arry.size()]);
    }

    public static Integer[] diguiRight (TreeNode root, ArrayList<Integer> arry) {
        if (root != null) {
            diguiRight(root.left, arry);
            diguiRight(root.right, arry);
                        arry.add(root.val);
        }
        return arry.toArray(new Integer[arry.size()]);
    }

}