例如:给定二叉树: [3,9,20,null,null,15,7],
    3   / \  9  20    /  \   15   7返回其层次遍历结果：
[  [3],  [9,20],  [15,7]]
来源：力扣（LeetCode）链接：https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {    public List<List<Integer>> levelOrder(TreeNode root) {        List<List<Integer>> ans = new ArrayList<>();        if (root == null) {            return ans;        }        Deque<TreeNode> deque = new ArrayDeque<>();        deque.offer(root);        while (!deque.isEmpty()) {            int size = deque.size();            List<Integer> temp = new ArrayList<>();            for (int i = 0; i < size; i++) {                TreeNode node = deque.poll();                if (node != null) {                    int val = node.val;                    temp.add(val);                    if (node.left != null) {                        deque.offer(node.left);                    }                    if (node.right != null) {                        deque.offer(node.right);                    }                }            }            ans.add(temp);                    }        return ans;    }}