剑指offer 22. 从上往下打印二叉树

 1 import java.util.ArrayList;
 2 import java.util.Queue;
 3 import java.util.LinkedList;
 4 public class Solution {
 5     // 层序遍历二叉树即可
 6     public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
 7         Queue<TreeNode> Q = new LinkedList<>();
 8         ArrayList<Integer> list = new ArrayList<>();
 9         if(root == null){
10             return list;
11         }
12         Q.offer(root);
13         TreeNode top;
14         while(!Q.isEmpty()){
15             // 访问队首元素
16             top = Q.poll();
17             list.add(top.val);
18             if(top.left != null)
19                 Q.offer(top.left);
20             if(top.right != null)
21                 Q.offer(top.right);
22         }
23         return list;
24     }
25 }