题解 | #实现二叉树先序，中序和后序遍历# Java版本

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
     */
    public int[][] threeOrders (TreeNode root) {
        List<Integer> preList = new ArrayList<>();
        preOrder(root, preList);

        List<Integer> midList = new ArrayList<>();
        midOrder(root, midList);

        List<Integer> lastList = new ArrayList<>();
        lastOrder(root, lastList);
        
        //最后的整合
        int length = preList.size();
        int[] preArr = preList.stream().mapToInt(Integer::valueOf).toArray();
        int[] midArr = midList.stream().mapToInt(Integer::valueOf).toArray();
        int[] lastArr = lastList.stream().mapToInt(Integer::valueOf).toArray();
        
        int[][] result = new int[3][length];
        result[0] = preArr;
        result[1] = midArr;
        result[2] = lastArr;
        return result;
    }
    
    /** 先序遍历 */
    public void preOrder(TreeNode root,List<Integer> list) {
        if(root==null) {
            return;
        }
        list.add(root.val);
        preOrder(root.left, list);
        preOrder(root.right, list);
    }
    
    /** 中序遍历 */
    public void midOrder(TreeNode root,List<Integer> list) {
        if(root==null) {
            return;
        }
        midOrder(root.left, list);
        list.add(root.val);
        midOrder(root.right, list);
    }
    
    /** 后序遍历 */
    public void lastOrder(TreeNode root,List<Integer> list) {
        if(root==null) {
            return;
        }
        lastOrder(root.left, list);
        lastOrder(root.right, list);
        list.add(root.val);
    }
}