例如:给定二叉树: [3,9,20,null,null,15,7],
    3   / \  9  20    /  \   15   7
返回：
[3,9,20,15,7]

提示：
节点总数 <= 1000
来源：力扣（LeetCode）链接：https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-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 int[] levelOrder(TreeNode root) {        List<Integer> nodeValList = new ArrayList<>();        bfs(root, nodeValList);        int size = nodeValList.size();        int[] ans = new int[size];        for (int i = 0; i < size; i++) {            ans[i] = nodeValList.get(i);        }        return ans;    }
    public void bfs(TreeNode root, List<Integer> ans) {        if (root == null) {            return;        }        Queue<TreeNode> queue = new LinkedList<>();        queue.add(root);        while (!queue.isEmpty()) {            int size = queue.size();            for (int i = 0; i < size; i++) {                TreeNode node = queue.poll();                ans.add(node.val);                if (node.left != null) {                    queue.add(node.left);                }                if (node.right != null) {                    queue.add(node.right);                }            }        }    }}