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],
]

Analysis: BFS. 

/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
Queue<TreeNode> frontier = new LinkedList<TreeNode>();

if(root==null) return res;
frontier.add(root);
while(!frontier.isEmpty()) {
Queue<TreeNode> next = new LinkedList<TreeNode>();
ArrayList<Integer> cell = new ArrayList<Integer>();
for(int i=frontier.size()-1; i>=0; i--) {
TreeNode current = frontier.poll();
if(current.left!=null) next.add(current.left);
if(current.right!=null) next.add(current.right);
cell.add(current.val);
}
res.add(0, cell);
frontier = next;
}

return res;
}
}