剑指Offer(牛客网)-从上往下打印二叉树

题目来源：

https://www.nowcoder.com/practice/7fe2212963db4790b57431d9ed259701?tpId=13&tqId=11175&tPage=2&rp=1&ru=%2Fta%2Fcoding-interviews&qru=%2Fta%2Fcoding-interviews%2Fquestion-ranking

题目描述：

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。

代码如下：

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 PrintFromTopToBottom(TreeNode root) {
        ArrayList list = new ArrayList();
        if(root==null){
            return list;
        }
        Queue queue = new LinkedList();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode treeNode = queue.poll();
            if (treeNode.left != null) {
                queue.offer(treeNode.left);
            }
            if (treeNode.right != null) {
                queue.offer(treeNode.right);
            }
            list.add(treeNode.val);
        }
        return list;
    }
}