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 class Solution {
    /**
     * 
     * @param root TreeNode类 the root of binary tree
     * @return int整型二维数组
     */
    private ArrayList<Integer> preOrder;
    private ArrayList<Integer> postOrder;
    private ArrayList<Integer> inOrder;

    public void travel(TreeNode root) {
        if (root == null) {
            return;
        }
        preOrder.add(root.val);
        travel(root.left);
        inOrder.add(root.val);
        travel(root.right);
        postOrder.add(root.val);
    }

    public int[][] threeOrders (TreeNode root) {
        preOrder = new ArrayList<>();
        postOrder = new ArrayList<>();
        inOrder = new ArrayList<>();
        travel(root);
        int[][] result = new int[3][];
        result[0] = new int[preOrder.size()];
        result[1] = new int[preOrder.size()];
        result[2] = new int[preOrder.size()];
        for (int i = 0; i < preOrder.size(); i++) {
            result[0][i] = preOrder.get(i);
            result[1][i] = inOrder.get(i);
            result[2][i] = postOrder.get(i);
        }
        return result;
    }
}