[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].

/**
 * Definition for binary tree
 * 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 size = que.size();
            for(int i=0;i<size;i++)
            {
                TreeNode *node = que.front();
                // the first element in the queue
                // is the rightmost on that level
                if(0 == i)
                {
                    res.push_back(node->val);
                }
                if(node->right)
                {
                    que.push(node->right);
                }
                if(node->left)
                {
                    que.push(node->left);
                }
                que.pop();
            }
        }
        return res;
    }
};