Linq - 如何从平面转换为分层？

var employees = new List<Employee>
{
    new Employee { Id = 1, Name = "John", Department = "HR" },
    new Employee { Id = 2, Name = "Jane", Department = "IT" },
    new Employee { Id = 3, Name = "Mike", Department = "HR" },
    new Employee { Id = 4, Name = "Sarah", Department = "IT" }
};

var groupedEmployees = employees.GroupBy(e => e.Department);

foreach (var group in groupedEmployees)
{
    Console.WriteLine($"Department: {group.Key}");
    foreach (var employee in group)
    {
        Console.WriteLine($"- {employee.Name}");
    }
    Console.WriteLine();
}

Department: HR
- John
- Mike

Department: IT
- Jane
- Sarah