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
        int[][] arr = new int[3][];
        List preList = new ArrayList();
        List inList = new ArrayList();
        List postList = new ArrayList();
        myOrder(root, preList, inList, postList);
        arr[0] = listToArr(preList);
        arr[1] = listToArr(inList);
        arr[2] = listToArr(postList);
        return arr;
    }

   private static void myOrder(TreeNode root, List<Integer> listPre, List<Integer> listIn, List<Integer> listPost) {
        if(root != null){
            listPre.add(root.val);
            myOrder(root.left, listPre, listIn, listPost);
            listIn.add(root.val);
            myOrder(root.right, listPre, listIn, listPost);
            listPost.add(root.val);
        }
    }

   private static int[] listToArr(List<Integer> list){
        int[] arr = new int[list.size()];
        for(int i = 0; i < list.size(); i++){
            arr[i] = list.get(i);
        }
        return arr;
    }

//     private static void preOrder(TreeNode root, List<Integer> list){
//         if(root != null){
//             list.add(root.val);
//             preOrder(root.left, list);
//             preOrder(root.right, list);
//         }
//     }
//     private static void inOrder(TreeNode root, List<Integer> list){
//         if(root != null){
//             inOrder(root.left, list);
//             list.add(root.val);
//             inOrder(root.right, list);
//         }
//     }
//     private static void postOrder(TreeNode root, List<Integer> list){
//         if(root != null){
//             postOrder(root.left, list);
//             postOrder(root.right, list);
//             list.add(root.val);
//         }
//     }

}