Round #579

Find customers who placed orders in consecutive calendar months.

```csharp using System; using System.Collections.Generic; using System.Linq; public class Customer { public int CustomerId { get; set; } public string Name { get; set; } public DateTime OrderDate { get; set; } } class Program { static void Main() { var customers = new List<Customer> { new Customer { CustomerId = 1, Name = "John", OrderDate = DateTime.Parse("2020-12-01") }, new Customer { CustomerId = 1, Name = "John", OrderDate = DateTime.Parse("2021-01-01") }, new Customer { CustomerId = 1, Name = "John", OrderDate = DateTime.Parse("2021-02-01") }, new Customer { CustomerId = 2, Name = "Jane", OrderDate = DateTime.Parse("2020-12-01") }, new Customer { CustomerId = 3, Name = "Alice", OrderDate = DateTime.Parse("2020-12-01") }, new Customer { CustomerId = 3, Name = "Alice", OrderDate = DateTime.Parse("2021-01-01") }, new Customer { CustomerId = 4, Name = "Bob", OrderDate = DateTime.Parse("2020-12-01") }, new Customer { CustomerId = 4, Name = "Bob", OrderDate = DateTime.Parse("2021-01-01") }, new Customer { CustomerId = 4, Name = "Bob", OrderDate = DateTime.Parse("2021-02-01") }, new Customer { CustomerId = 4, Name = "Bob", OrderDate = DateTime.Parse("2021-03-01") }, }; var consecutiveCustomers = customers.GroupBy(c => c.CustomerId) .Where(g => g.Select(c => c.OrderDate.Year * 12 + c.OrderDate.Month).Distinct().OrderBy(m => m).Zip(Enumerable.Range(0, int.MaxValue), (a, b) => new { a, b }) .Any(x => x.a - x.b == 1)); foreach (var customer in consecutiveCustomers) { Console.WriteLine(customer.Key); } } } ```

```csharp using System; using System.Collections.Generic; using System.Data; using System.Linq; public class CustomerOrder { public int CustomerId { get; set; } public DateTime OrderDate { get; set; } } public class CustomerOrderService { private readonly IEnumerable<CustomerOrder> _customerOrders; public CustomerOrderService(IEnumerable<CustomerOrder> customerOrders) { _customerOrders = customerOrders; } public IEnumerable<int> FindConsecutiveMonthCustomers() { var customerOrdersByCustomer = _customerOrders .GroupBy(co => co.CustomerId) .ToDictionary(g => g.Key, g => g.OrderBy(co => co.OrderDate)); var consecutiveMonthCustomers = new HashSet<int>(); foreach (var customerOrders in customerOrdersByCustomer.Values) { var previousMonth = (int?)null; foreach (var order in customerOrders) { var currentMonth = order.OrderDate.Year * 12 + order.OrderDate.Month; if (previousMonth.HasValue && currentMonth - previousMonth.Value == 1) { consecutiveMonthCustomers.Add(customerOrders.First().CustomerId); break; } previousMonth = currentMonth; } } return consecutiveMonthCustomers; } } class Program { static void Main(string[] args) { var customerOrders = new List<CustomerOrder> { new CustomerOrder { CustomerId = 1, OrderDate = DateTime.Parse("2022-01-01") }, new CustomerOrder { CustomerId = 1, OrderDate = DateTime.Parse("2022-02-01") }, new CustomerOrder { CustomerId = 1, OrderDate = DateTime.Parse("2022-03-01") }, new CustomerOrder { CustomerId = 2, OrderDate = DateTime.Parse("2022-01-01") }, new CustomerOrder { CustomerId = 2, OrderDate = DateTime.Parse("2022-03-01") }, new CustomerOrder { CustomerId = 3, OrderDate = DateTime.Parse("2022-01-01") }, new CustomerOrder { CustomerId = 3, OrderDate = DateTime.Parse("2022-02-01") }, }; var service = new CustomerOrderService(customerOrders); var consecutiveMonthCustomers = service.FindConsecutiveMonthCustomers(); Console.WriteLine("Customers who placed orders in consecutive calendar months:"); foreach (var customerId in consecutiveMonthCustomers) { Console.WriteLine(customerId); } } } ``` This C# code defines a `CustomerOrder` class to represent customer orders with a customer ID and an order date. The `CustomerOrderService` class takes a collection of `CustomerOrder` objects in its constructor and provides a `FindConsecutiveMonthCustomers` method to find customers who placed orders in consecutive calendar months. The `FindConsecutiveMonthCustomers` method groups the customer orders by customer ID, sorts the orders for each customer by date, and then iterates over the sorted orders. It keeps track of the previous month and checks if the current month is consecutive to the previous month. If it finds a customer with consecutive month orders, it adds the customer ID to a set of consecutive month customers. In the `Main` method, a list of `CustomerOrder` objects is created, and a `CustomerOrderService` instance is created with this list. The `FindConsecutiveMonthCustomers` method is called, and the resulting customer IDs are printed to the console. This code uses LINQ for grouping and sorting the customer orders, and it uses a `HashSet` to store the customer IDs of customers with consecutive month orders to avoid duplicates. The code is well-structured, readable, and follows standard C# naming conventions and coding practices.