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。

import java.util.ArrayList;
import java.util.Queue;
import java.util.LinkedList;

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

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

    }

}
*/
public class Solution {
    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {

        ArrayList<ArrayList<Integer> > List=new ArrayList<ArrayList<Integer> >();
        
        Queue <TreeNode> queue =new LinkedList<>();
        if(pRoot==null)
            return List;
        queue.offer(pRoot);
        while(!queue.isEmpty())
        {
            int size=queue.size();
            ArrayList<Integer> list=new ArrayList<>();
            while(size>0)
            {
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left!=null) queue.offer(node.left);
                if(node.right!=null) queue.offer(node.right);
                size--;
            }
            List.add(list);
        }
        
        return List;
    }
    
}