32 JZOF 】 【 print down on binary tree

不分行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.例如输入{8,6,10,#,#,2,1},如以下图中的示例二叉树,则依次打印8,6,10,2,1(空节点不打印,跳过),请你将打印的结果存放到一个数组里面,返回.

by means of a queue.

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