LeetCode Binary Tree Right Side View

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

For example:
Given the following binary tree,

   1            <---

 /   \

2     3         <---

 \     \

  5     4       <---

 

You should return [1, 3, 4].

Credits:
Special thanks to @amrsaqr for adding this problem and creating all test cases.

 1 /**

 2  * Definition for a binary tree node.

 3  * struct TreeNode {

 4  *     int val;

 5  *     TreeNode *left;

 6  *     TreeNode *right;

 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 8  * };

 9  */

10 class Solution {

11 public:

12     vector<int> rightSideView(TreeNode* root) {

13         vector<int> res;

14         if (root == NULL) {

15             return res;

16         }

17         queue<TreeNode*> que;

18         que.push(root);

19         while (!que.empty()) {

20             int len = que.size();

21             res.push_back(que.back()->val);

22             for (int i=0; i<len; i++) {

23                 TreeNode* node = que.front();

24                 que.pop();

25                 if (node->left != NULL) {

26                     que.push(node->left);

27                 }

28                 if (node->right != NULL) {

29                     que.push(node->right);

30                 }

31             }

32         }

33         return res;

34     }

35 };

会不会有更简单的方法呢