107. 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}

,

3
/ \
9  20
/  \
15   7

return its bottom-up level order traversal as:

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

confused what 

"{1,#,2,3}"

 means? >
read more on how binary tree is serialized on OJ.

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<TreeNode> tree1 =new ArrayList <TreeNode> ();
List<List<Integer>> ret = new ArrayList <List<Integer>> ();
List<List<Integer>> ret2 = new ArrayList <List<Integer>> ();
if(root==null)return ret;
tree1.add(root);
while(tree1.size()>0){
List<TreeNode> tree2 =new ArrayList <TreeNode> ();
List <Integer> a = new ArrayList <Integer> ();
for(int i = 0;i<tree1.size();i++){
a.add(tree1.get(i).val);
if(tree1.get(i).left!=null)tree2.add(tree1.get(i).left);
if(tree1.get(i).right!=null)tree2.add(tree1.get(i).right);
}
tree1=tree2;
ret2.add(a);
}
for(int i=ret2.size()-1;i>=0;i--){
ret.add(ret2.get(i));
}
return ret;
}
}