Round #796

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.Linq; public class CustomerOrder { public int CustomerId { get; set; } public DateTime OrderDate { get; set; } } public class Program { public static List<int> FindConsecutiveCustomers(List<CustomerOrder> orders) { // Group orders by customer and order date var customerOrders = orders.GroupBy(o => o.CustomerId) .ToDictionary(g => g.Key, g => g.Select(o => o.OrderDate).OrderBy(d => d).ToList()); // Find customers with consecutive orders var consecutiveCustomers = new List<int>(); foreach (var customer in customerOrders) { for (int i = 0; i < customer.Value.Count - 1; i++) { if (customer.Value[i + 1].Month == customer.Value[i].Month + 1 || (customer.Value[i].Month == 12 && customer.Value[i + 1].Month == 1 && customer.Value[i].Year + 1 == customer.Value[i + 1].Year)) { consecutiveCustomers.Add(customer.Key); break; } } } return consecutiveCustomers; } public static void Main(string[] args) { var orders = 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 = 2, OrderDate = DateTime.Parse("2022-01-01") }, new CustomerOrder { CustomerId = 2, OrderDate = DateTime.Parse("2022-03-01") }, new CustomerOrder { CustomerId = 3, OrderDate = DateTime.Parse("2022-12-01") }, new CustomerOrder { CustomerId = 3, OrderDate = DateTime.Parse("2023-01-01") }, }; var consecutiveCustomers = FindConsecutiveCustomers(orders); Console.WriteLine(string.Join(", ", consecutiveCustomers)); } } ``` This code defines a `CustomerOrder` class to represent customer orders and a `FindConsecutiveCustomers` method to find customers who placed orders in consecutive calendar months. The `Main` method demonstrates how to use this method with a sample list of orders.