如何计算任意数量的链接子对象？

Question

I apologize if my direct question is unclear, I only know how to explain this through the following example.

I'm trying to calculate the amount of child objects connected to an object. Each object only holds the children that are connected directly

I have the following classes:

class Question 
{
    public int ID { get; set; } //Primary key
    public string Text { get; set; }
    public string[] Options { get; set; }
    public string Answer { get; set; }


    public List<Link> Links { get; set; }
}

class Link 
{
    public int ID { get; set; }
    public string Answer { get; set; }
    public int QuestionID { get; set; } //Foreign key
}

I use these classes to generate the following dialog editor (see image).

Example: If I'm at node AB , how can I figure out how many questions are linked underneath it? Which in this case is ABA and ABAA ? So the correct answer would be 2. All nodes connected to port Answer B and Answer C should be counted too, should they exist.

I've had little success in coming up with idea's.

I use something like the following function to get all direct children of a single question:

public List<Question> GetDirectChildren(Question question) 
{
    var directChildren = new List<Question>();

    foreach(Link link in question.Links)
        directChildren.Add(myQuestions.FirstOrDefault(f => f.ID == link.QuestionID));

    return directChildren;
}

To get all child-child objects, I would create a nested loop, and another nested loop for each children underneath that. But the problem I can't solve is that there can be an infinite amount of linked objects.

Is there a structural way to iterate through or count all linked objects of an object in this scenario? With the purpose of retrieving the count of linked objects.

Answer 1

You have a tree there which is a very common data structure. Since in trees, every child of a node is also a tree, you usually use recursion to navigate through the tree. This is also called tree traversal .

In your case, when you just want to count the nodes in your tree, you just have to break it down to a simpler problem (in general, that's a very common approach when solving difficult problems):

When you have a tree, which has no children, then the number of nodes is very clearly defined: It's one. Now, when you add one level, how many nodes do you have? One for the parent, and one for every child.

Since a child is also a tree, you can now generalize this: For any tree, the number of nodes in the tree is one plus the number of nodes for every child tree.

That is your recursive definition, which you can then use to create a recursive function to calculate the total number of nodes in any tree:

public int CountNodes(Node tree)
{
    int count = 1; // current node

    foreach (var child in tree.Children)
    {
        count += CountNodes(child);
    }

    return count;
}

Once you understand this, it's also not a lot more complicated to do different things while traversing the tree. For example, instead of counting the nodes, you could collect them all in a list; or eg print a visual representation of your tree.

Answer 2

You can recursively call your method GetDirectChildren on all the Link inside your question.

public List<Question> GetDirectChildren(Question question) 
{
    var children = new List<Question>();

    if (question.Links.Count > 0)
    {
        foreach(Link link in question.Links)
        {
            Question curQuestion = myQuestions.FirstOrDefault(f => f.ID == link.QuestionID);
            children.Add(curQuestion);
            children.AddRange(GetDirectChildren(curQuestion));
        }
    }
    else
        return children;
}

But if there's at least one cyclical structure, this code will not work.