Big O of a BST recusive size function.

[trevormoon]

Hey guys I'm trying to figure out what the big O of my recurisve_size function would be.

For this function would the O be O(n) since the function has to go through all the nodes in order to find the size?

template <class T>
int Binary_tree<T>::recursive_size(Binary_node<T> *sub_root) const
{
  if(sub_root ==NULL)
    return 0;
    else
       return 1 + recursive_size(sub_root->left) + recursive_size(sub_root->right);
}