剑指Offer 从上往下打印二叉树

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