给定一个二叉树，返回该二叉树层序遍历的结果，（从左到右，一层一层地遍历）

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        if (root == null) {
            return new ArrayList<ArrayList<Integer>>();
        }
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        // 每一层以null结尾
        queue.offer(null);
        // 遍历到最后一层时，只剩下null
        while (!queue.isEmpty() && queue.peek() != null) {
            ArrayList<Integer> r  = new ArrayList<>();
            while (queue.peek() != null) {
                TreeNode t = queue.poll();
                r.add(t.val);
                if (t.left != null) {
                    queue.offer(t.left);
                }
                if (t.right != null) {
                    queue.offer(t.right);
                }
            }
            // 需要弹出上一层队尾的null
            queue.poll();
            queue.offer(null);
            res.add(r);
        }
        return res;
    }
}