# 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]
]
``````

## Solution

Queue version. On the basis of 'Binary Tree Level Order Traversal', reverse the final vector.

## Code

``````/**
* 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<List<Integer>> res = new ArrayList<List<Integer>>();
if (root == null) return res;
q.offer(root);
q.offer(null);
List<Integer> level = new ArrayList<Integer>();

while(true) {
TreeNode node = q.poll();
if (node != null) {
if(node.left!=null) q.offer(node.left);
if(node.right!=null) q.offer(node.right);
} else {