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) {
        // write code here
        ArrayList<Integer> arr1 = new ArrayList<>();
        ArrayList<Integer> arr2 = new ArrayList<>();
        ArrayList<Integer> arr3 = new ArrayList<>();

        if (root != null) {
            first(root, arr1);
            mid(root, arr2);
            behind(root, arr3);
        }

        int[] res1 = new int[arr1.size()];
        for (int i = 0; i < arr1.size(); i++) {
            res1[i] = arr1.get(i);
        }
        int[] res2 = new int[arr2.size()];
        for (int i = 0; i < arr2.size(); i++) {
            res2[i] = arr2.get(i);
        }
        int[] res3 = new int[arr3.size()];
        for (int i = 0; i < arr3.size(); i++) {
            res3[i] = arr3.get(i);
        }

        return new int[][] {res1, res2, res3};
    }

    public void first(TreeNode root, ArrayList<Integer> arr) {
        if (root != null) {
            arr.add(root.val);
            first(root.left, arr);
            first(root.right, arr);
        }
    }

    public void mid(TreeNode root, ArrayList<Integer> arr) {
        if (root != null) {
            mid(root.left, arr);
            arr.add(root.val);
            mid(root.right, arr);
        }
    }

    public void behind(TreeNode root, ArrayList<Integer> arr) {
        if (root != null) {
            behind(root.left, arr);
            behind(root.right, arr);
            arr.add(root.val);
        }
    }
}