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public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        ArrayList<ArrayList<Integer>> ans = new ArrayList<>();

		if(root==null){
			return ans;
		}
		Queue<TreeNode> queue = new ArrayDeque<TreeNode>();
		queue.add(root);

		while(!queue.isEmpty()){
			ArrayList<Integer> list = new ArrayList<>();

			int n = queue.size();

			for(int i=0;i<n;i++){
				TreeNode cur = queue.poll();
				list.add(cur.val);

				if (cur.left!=null) {
					queue.add(cur.left);
				}
				
				if(cur.right!=null){
					queue.add(cur.right);
				}
			}
			ans.add(list);
		}

		return ans;
    }