牛客网-从上到下打印二叉树

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 {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        
      ArrayList<Integer> list = new ArrayList<>();

        if(root==null)
            return list;

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        //队列一直不为空的时候执行
        while (!queue.isEmpty()){
            //取出来队列先入队列的顶的元素
            list.add(queue.peek().val);

            //取出队列顶元素
            TreeNode poll = queue.poll();

            //
            if(poll.left!=null)
                queue.offer(poll.left);
            if(poll.right!=null)
                queue.offer(poll.right);
        }
        
        return list;
        
    }
}