从上往下打印出二叉树的每个节点，同层节点从左至右打印。

import java.util.ArrayList;

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

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

}

}

public class Solution {
public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
ArrayList<Integer> tree = new ArrayList<Integer>();
if(root==null){
return list;
}
Queue<TreeNode> treeNode = new LinkList<TreeNode>();
treeNode.offer(root);
while(!treeNode.isEmpty()){
TreeNode node = treeNode.poll();
if(node.left!=null)
{
treeNode.offer(node.left);

}
if(node.right!=null)
{
treeNode.offer(node.right);

}
tree.add(node.val);

}
return tree;
}
}