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 TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @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
        ArrayList<Integer> pre = new ArrayList<Integer>();
        preOrder(root,pre);

        ArrayList<Integer> in = new ArrayList<Integer>();
        inOrder(root,in);

          ArrayList<Integer> post = new ArrayList<Integer>();
        postOrder(root,post);
       
        int[][] ans = new int[][] {
           pre.stream().mapToInt(i -> i).toArray(),
            in.stream().mapToInt(i->i).toArray(),
            post.stream().mapToInt(i->i).toArray()
        };

        return ans;
    }
    public void preOrder(TreeNode root,ArrayList<Integer> ans)  {
        if (root != null) {
        ans.add(root.val);
        
        preOrder(root.left,ans);
        preOrder(root.right,ans);
}
        
    }

        public void inOrder(TreeNode root,ArrayList<Integer> ans)  {
        if (root != null) {

        inOrder(root.left,ans);

        ans.add(root.val);
       
        inOrder(root.right,ans);

         }
    }

        public void postOrder(TreeNode root,ArrayList<Integer> ans)  {
        if (root != null) {
        
        postOrder(root.left,ans);
        postOrder(root.right,ans);
        ans.add(root.val);
        }
           
        

        
    }
}