从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。

Posted 2021-12-18 q-1993

import java.util.ArrayList;


/*
public class TreeNode 
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) 
        this.val = val;

    


*/
import java.util.*;
public class Solution 
    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) 
        ArrayList<ArrayList<Integer>> arrayList = new ArrayList<ArrayList<Integer>>();
        if(pRoot == null)
            return arrayList;
        
        Queue<TreeNode> q1 = new LinkedList<TreeNode>();
        q1.add(pRoot);
        Queue<TreeNode> q2 = new LinkedList<TreeNode>();
        while(!q1.isEmpty() || !q2.isEmpty())
            if(!q1.isEmpty())
                ArrayList<Integer> list = new ArrayList<Integer>();
                while(!q1.isEmpty())
                   TreeNode node = q1.poll();
                    list.add(node.val);
                    if(node.left != null)
                        q2.add(node.left);
                    
                    if(node.right != null)
                        q2.add(node.right);
                    
                
                arrayList.add(list);
            else
                ArrayList<Integer> list = new ArrayList<Integer>();
                while(!q2.isEmpty())
                    TreeNode node = q2.poll();
                    list.add(node.val);
                    if(node.left != null)
                        q1.add(node.left);
                    
                    if(node.right != null)
                        q1.add(node.right);
                    
                
                arrayList.add(list);
            
        
        return arrayList;