Sum of the nodes of binary tree

Question: Write a function to calculate the sum of binary search tree nodes?
Time Complexity: O(n)

int
sum_of_tree_nodes(tree *t)
{
    if (t == NULL) {
        return 0;
    } else {
        return (sum_of_tree_nodes(t->right) +
                t->data                     +
                sum_of_tree_nodes(t->left));
    }
}