[Leetcode] Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

Solution:

在I的基础上倒过来就好。

 1 /**
 2  * Definition for binary tree
 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>> ret=new ArrayList<List<Integer>>();
13         if(root==null)
14             return ret;
15         List<List<Integer>> alal=new ArrayList<List<Integer>>();
16         List<Integer> al=new ArrayList<Integer>();
17         Queue<TreeNode> queue=new LinkedList<TreeNode>();
18         queue.add(root);
19         int curLevel=1;
20         int nextLevel=0;
21         
22         while(!queue.isEmpty()){
23             TreeNode cur=queue.remove();
24             al.add(cur.val);
25             curLevel--;
26             
27             if(cur.left!=null){
28                 queue.add(cur.left);
29                 nextLevel++;
30             }
31             
32             if(cur.right!=null){
33                 queue.add(cur.right);
34                 nextLevel++;
35             }
36             
37             if(curLevel==0){
38                 alal.add(al);
39                 al=new ArrayList<Integer>();
40                 curLevel=nextLevel;
41                 nextLevel=0;
42             }
43         }
44         for(int i=alal.size()-1;i>=0;--i){
45             ret.add(alal.get(i));
46         }
47         
48         return ret;
49     }
50 }