import java.util.*;

/*

 * public class TreeNode {

 *   int val = 0;

 *   TreeNode left = null;

 *   TreeNode right = null;

 * }

 */

public class Solution {

    ArrayList<ArrayList<Integer>> res = new ArrayList<>();

    /**

     *

     * @param root TreeNode类

     * @return int整型ArrayList<ArrayList<>>

     */

    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {

        level(root);

        return res;

    }

    public void level(TreeNode root) {

        Queue<TreeNode> queue = new LinkedList<>();

        List<Integer> list = new ArrayList<>();

        if(root == null) {

            return;

        }

        queue.add(root);

        while(!queue.isEmpty()) {

            //需要定义每一层元素的个数（队内元素个数）

            int size = queue.size();

            while(size > 0) {

                TreeNode node = queue.poll();

                list.add(node.val);

                if(node.left != null) {

                    queue.add(node.left);

                }

                if(node.right != null) {

                    queue.add(node.right);

                }

                size--;

            }

            res.add(new ArrayList<>(list));

            list = new ArrayList<>();

        }

    }

}