Binary Tree Level Order Traversal II

This is very similar to Binary Tree Level Order Traversal. Just reverse the order of the list. 
ArrayList has API- > add(int index, Element) or Collections.reverse(rst)

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
 
 
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: buttom-up level order a list of lists of integer
     */
    public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
        // write your code here
        ArrayList<ArrayList<Integer>> rst = new ArrayList<ArrayList<Integer>>();
        if (root == null) {
            return rst;
        }
        
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root);
        while (!q.isEmpty()) {
            ArrayList<Integer> level = new ArrayList<Integer>();
            int n = q.size();
            for (int i = 0; i < n; i++) {
                TreeNode node = q.poll();
                level.add(node.val);
                if (node.left != null) {
                    q.offer(node.left);
                }
                if (node.right != null) {
                    q.offer(node.right);
                }
            }
            rst.add(0, level);
        }
        return rst;
    }
}