題目：從上往下打印出二叉樹的每個節點，同層節點從左至右列印。

思路：其實就是二叉樹的層次遍歷問題，藉助於佇列來實現對二叉樹的層次遍歷

程式碼：

import java.util.ArrayList;
import java.util.LinkedList;
/*class TreeNode 
{
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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

    }
}*/
public class LevelTest
{
   public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) 
   {
	ArrayList<Integer> list  = new ArrayList<Integer>();
        if(root == null)
           return list;
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        TreeNode current = null;
        queue.offer(root);
        while(!queue.isEmpty())
        {
        	current = queue.poll();//出隊當前根節點
        	list.add(current.val);//將根節點元素的值新增到list中
        	if(current.left != null) 
        		queue.offer(current.left);
        	if(current.right != null)
        		queue.offer(current.right);
        }
        return list;
   
   }
}