/**
 * 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
        if(root == null)
            return new ArrayList<>();
        
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        
        Queue<TreeNode> list = new LinkedList<>();
        
        list.offer(root);
        
        while(!list.isEmpty()) {
            int size = list.size();
            ArrayList<Integer> level = new ArrayList<>();
            for(int i=0;i<size;i++) {
                TreeNode cur = list.poll();
                level.add(cur.val);
                if(cur.left != null)
                    list.offer(cur.left);
                if(cur.right != null)
                    list.offer(cur.right);
            }
            res.add(level);
        }
        Collections.reverse(res);
        return res;
    }
}