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
     */
    public int[][] threeOrders (TreeNode root) {
        // write code here
        List<Integer> list1 = new ArrayList<>();
        List<Integer> list2 = new ArrayList<>();
        List<Integer> list3 = new ArrayList<>();
        
        preOrder(root, list1);
        inOrder(root, list2);
        postOrder(root, list3);
        
        int[][] res = new int[3][list1.size()];
        res[0] = list1.stream().mapToInt(Integer::valueOf).toArray();
        res[1] = list2.stream().mapToInt(Integer::valueOf).toArray();
        res[2] = list3.stream().mapToInt(Integer::valueOf).toArray();
        return res;
    }
    
    private static void preOrder(TreeNode root, List<Integer> list){
        if(root == null){
            return;
        }
        
        list.add(root.val);
        preOrder(root.left, list);
        preOrder(root.right, list);
    }
    
    private static void inOrder(TreeNode root, List<Integer> list){
        if(root == null){
            return;
        }
        
        inOrder(root.left, list);
        list.add(root.val);
        inOrder(root.right, list);
    }
    
    private static void postOrder(TreeNode root, List<Integer> list){
        if(root == null){
            return;
        }
        
        postOrder(root.left, list);
        postOrder(root.right, list);
        list.add(root.val);
    }
}