每天一道剑指offer-从上往下打印二叉树

public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> resultList = new ArrayList<>();
        if(root == null)
            return resultList;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(queue.size() != 0)//队列不为空一直进行
        {
            TreeNode tempRoot = queue.poll();//出队
            if(tempRoot.left != null)//左子节点不为空，左子节点入队
                queue.offer(tempRoot.left);
            if(tempRoot.right != null)//右子节点不为空，右子节点入队
                queue.offer(tempRoot.right);
            resultList.add(tempRoot.val);//把出队的节点的值保留下来
        }
        return resultList;
    }
}