❔ How to find the value in the tree?

public class DepartmentDigestDto
{
    [JsonProperty( PropertyName = "id" )]
    [JsonRequired]
    public string Id { get; set; }

    [JsonProperty( PropertyName = "title" )]
    public string Title { get; set; }

    [JsonProperty( PropertyName = "parent_department_id" )]
    public string ParentDepartmentId { get; set; }
}

static internal class Extensions
{
    static public IEnumerable<ExtensionTreeItem<T>> GenerateTree<T, K>(
        this IEnumerable<T> collection,
        Func<T, K> idSelector,
        Func<T, K> parentIdSelector,
        K rootId = default( K ) )
    {
        foreach ( var item in collection
            .Where( item => EqualityComparer<K>.Default
                .Equals( parentIdSelector( item ), rootId ) ) )
        {
            yield return new ExtensionTreeItem<T>
            {
                Item = item,
                Children = collection.GenerateTree(
                    idSelector,
                    parentIdSelector,
                    idSelector( item ) )
            };
        }
    }
}

internal class ExtensionTreeItem<T>
{
    public T Item { get; set; }
    public IEnumerable<ExtensionTreeItem<T>> Children { get; set; }
}

var departmentsTree = departments
    .GenerateTree(
        department => department.Id,
        department => department.ParentDepartmentId )
    .ToDictionary( x => x.Item.Id, x => x.Children );