I learned this method from someone's solution in the Binary Tree Level Order Traversal

```
class Solution {
public:
vector<int> rightSideView(TreeNode* root) {
vector<int> res;
if (root == NULL) {
return res;
}
queue<TreeNode *> q;
q.push(root);
q.push(NULL);
while(!q.empty()) {
TreeNode *temp = q.front();
q.pop();
if (temp->left) {
q.push(temp->left);
}
if (temp->right) {
q.push(temp->right);
}
if (!q.front()) { // if see a marker, means that this level have ended
q.pop();
if (!q.empty()) {
q.push(NULL);
}
res.push_back(temp->val);
}
}
return res;
}
};
```