劍指Offer----從上往下打印二叉樹

從上往下打印出二叉樹的每個節點，同層節點從左至右打印。

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
/**
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> arrayList = new ArrayList<Integer>();
        if (root==null){
            return arrayList;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        while (!queue.isEmpty()){
            TreeNode node= queue.poll();
            arrayList.add(node.val);
            if (node.left!=null){
                queue.add(node.left);
            }
            if (node.right!=null){
                queue.add(node.right);
            }
        }
        return arrayList;
    }
}