从上到下打印二叉树 III

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution {
11     public List<List<Integer>> levelOrder(TreeNode root) {
12         if(root == null){
13             return new ArrayList<>();
14         }
15         List<List<Integer>> ret = new ArrayList<>();
16         Queue<TreeNode> queue = new LinkedList<>();
17         queue.add(root);
18         boolean d = true;
19         while(!queue.isEmpty()){
20             Queue<TreeNode> t = new LinkedList<>();
21             List<Integer> sub = new ArrayList<>();
22             while(!queue.isEmpty()){
23                 TreeNode node = queue.poll();
24                 sub.add(node.val);
25                 if(node.left != null){
26                     t.add(node.left);
27                 }
28                 if(node.right != null){
29                     t.add(node.right);
30                 }
31             }
32             if(d){
33                 ret.add(sub);
34             }else{
35                 List<Integer> tt = new ArrayList<>();
36                 for(int i=sub.size()-1;i>=0;i--){
37                     tt.add(sub.get(i));
38                 }
39                 ret.add(tt);
40             }
41             d = !d;
42             queue.addAll(t);
43         }
44         return ret;
45     }
46 }