Build tree type list by recursively checking parent-child relationship C#

Question

I have One class that has a list of itself so it can be represented in a tree structure.

I am pulling a flat list of these classes and want to unflatten it.

Answer 1

I have no idea why you want your BuildTree method return List - tree needs to have root node, so you should expect it to return single Group element, not a list.

I would create an extension method on IEnumerable:

public static class GroupEnumerable
{
    public static IList BuildTree(this IEnumerable source)
    {
        var groups = source.GroupBy(i => i.ParentID);

        var roots = groups.FirstOrDefault(g => g.Key.HasValue == false).ToList();

        if (roots.Count > 0)
        {
            var dict = groups.Where(g => g.Key.HasValue).ToDictionary(g => g.Key.Value, g => g.ToList());
            for (int i = 0; i < roots.Count; i++)
                AddChildren(roots[i], dict);
        }

        return roots;
    }

    private static void AddChildren(Group node, IDictionary> source)
    {
        if (source.ContainsKey(node.ID))
        {
            node.Children = source[node.ID];
            for (int i = 0; i < node.Children.Count; i++)
                AddChildren(node.Children[i], source);
        }
        else
        {
            node.Children = new List();
        }
    }
}

Usage

var flatList = new List() {
    new Group() { ID = 1, ParentID = null },    // root node
    new Group() { ID = 2, ParentID = 1 },
    new Group() { ID = 3, ParentID = 1 },
    new Group() { ID = 4, ParentID = 3 },
    new Group() { ID = 5, ParentID = 4 },
    new Group() { ID = 6, ParentID = 4 }
};


var tree = flatList.BuildTree();