问如何使用列表中的Linq搜索分层数据

EN

void Main()
{
    var distributors = new List<Distributor> {
        new Distributor { Id =  1, ParentId =  0, Name = "A" },
        new Distributor { Id =  7, ParentId =  1, Name = "B" },
        new Distributor { Id =  9, ParentId =  7, Name = "C" },
        new Distributor { Id = 13, ParentId =  9, Name = "D" },
        new Distributor { Id = 28, ParentId = 13, Name = "E" },
    };

    var valid = IsValidToAdd(distributors, 9);
    
    Console.WriteLine(valid);
}

public bool IsValidToAdd(List<Distributor> distributors, int parentId)
{
    var referCount = distributors.Count(d => d.ParentId == parentId);
    Console.WriteLine($"refer:{referCount}");
    if (referCount == 3)
    {
        Console.WriteLine("There are already 3 referals for this parent");
        return false;
    }

    var level = GetLevel(distributors, parentId);
    Console.WriteLine($"level: {level}");
    if (level > 5)
    {
        Console.WriteLine("There are already 5 levels of referals");
        return false;
    }
    
    return true;
}

public int GetLevel(List<Distributor> distributors, int parentId)
{
    var parent = distributors.FirstOrDefault(d => d.Id == parentId);

    if (parent == null)
    {
        return 1;
    }

    return 1 + GetLevel(distributors, parent.ParentId);
}

public class Distributor
{
    public int Id { get; set; }
    public int ParentId { get; set; }
    public string Name { get; set; }
}