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.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> rightSideView(TreeNode* root) {
        vector<int> res;
        if (root == NULL) {
            return res;
        }
        queue<TreeNode*> que;
        que.push(root);
        while (!que.empty()) {
            int len = que.size();
            res.push_back(que.back()->val);
            for (int i=0; i<len; i++) {
                TreeNode* node = que.front();
                que.pop();
                if (node->left != NULL) {
                    que.push(node->left);
                }
                if (node->right != NULL) {
                    que.push(node->right);
                }
            }
        }
        return res;
    }
};

会不会有更简单的方法呢