template: find the customer who bought most of each product

bought_most.cxx

struct Customer {
  long id;
  string const name;
  // ...
};

struct Order {
  long customer_id;
  long product_id;
  int quantity;
  // ...
};

/*
 * For each product ID that was purchased at least once, prints the product
 * ID and the name of the customer who ordered the largest total quantity.
 */
void bought_most(vector<Customer> const& customers, vector<Order> const& orders) {

}