题解 | #实现二叉树先序，中序和后序遍历#

记录

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
     */
    List<Integer> pre = new ArrayList<Integer>();
    List<Integer> mid = new ArrayList<Integer>();
    List<Integer> beh = new ArrayList<Integer>();
    public int[][] threeOrders (TreeNode root) {
        // write code here
        preOrder(root);
        midOrder(root);
        behOrder(root);
        int[][] res = new int[3][pre.size()];
        for (int i = 0; i < pre.size(); i++){
            res[0][i] = pre.get(i);
            res[1][i] = mid.get(i);
            res[2][i] = beh.get(i);
        }
        return res;


    }

     public void preOrder(TreeNode root){
         if (root == null){
             return;
         }
         pre.add(root.val);
         preOrder(root.left);
         preOrder(root.right);
     }
     public void midOrder(TreeNode root){
         if (root == null){
             return;
         }

         midOrder(root.left);
         mid.add(root.val);
         midOrder(root.right);
     }
     public void behOrder(TreeNode root){
         if (root == null){
             return;
         }

         behOrder(root.left);
         behOrder(root.right);
         beh.add(root.val);
     }
}