【LeetCode】107. Binary Tree Level Order Traversal II-二叉树水平顺序遍历2

二、思路：

三、代码：

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 public class Solution {
11     public List<List<Integer>> levelOrderBottom(TreeNode root) {
12         List<List<Integer>> list1 = new ArrayList<List<Integer>>();
13         List<List<Integer>> list3 = new ArrayList<List<Integer>>();
14         TreeNode node;
15         Queue<TreeNode> queue = new LinkedList<TreeNode>();
16         if(root==null){
17             return list1;
18         }
19         queue.offer(root);
20         while(!queue.isEmpty()){
21             List<Integer> list2 = new ArrayList<Integer>();
22             int size = queue.size();
23             for(int i=0;i<size;i++){
24                 node = queue.remove();
25                 list2.add(node.val);
26                 if(node.left!=null){
27                     queue.offer(node.left);
28                 }
29                 if(node.right!=null){
30                     queue.offer(node.right);
31                 }
32             }
33             list1.add(list2);
34         }
35
36         int size = list1.size()-1;
37         while(size>=0){
38             list3.add(list1.get(size));
39             size--;
40         }
41         return list3;
42     }
43 }