简单粗暴

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    private List<TreeNode> preOrder = new ArrayList<>();
    private List<TreeNode> inOrder = new ArrayList<>();
    private List<TreeNode> postOrder = new ArrayList<>();
    /**
     * 
     * @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
        pre(root);
        in(root);
        post(root);
        int[][] res = new int[3][preOrder.size()];
        for (int i = 0; i < preOrder.size(); i++) {
            res[0][i] = preOrder.get(i).val;
        }
        for (int i = 0; i < inOrder.size(); i++) {
            res[1][i] = inOrder.get(i).val;
        }
        for (int i = 0; i < postOrder.size(); i++) {
            res[2][i] = postOrder.get(i).val;
        }
        return res;
    }
    public void pre(TreeNode root) {
        if (root == null) return;
        preOrder.add(root);
        pre(root.left);
        pre(root.right);
    }
    public void in(TreeNode root) {
        if (root == null) return;
        in(root.left);
        inOrder.add(root);
        in(root.right);
    }
    public void post(TreeNode root) {
        if (root == null) return;
        post(root.left);
        post(root.right);
        postOrder.add(root);
    }
}