The following is my AC solution Binary Tree Level Order Traversal II (II). It is only one difference from Binary Tree Level Order Traversal I (I). Note that for II, we do result.add(0, level), so every time the level is added at the beginning (index 0). For I, we only change this line to result.add(level). Simple!

```
public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List result = new ArrayList();
if (root == null) {
return result;
}
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while (!queue.isEmpty()) {
ArrayList<Integer> level = new ArrayList();
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode head = queue.poll();
level.add(head.val);
if (head.left != null) {
queue.offer(head.left);
}
if (head.right != null) {
queue.offer(head.right);
}
}
result.add(0, level);
}
return result;
}
}
```