Building Ordering a tree as a list in C#

Question

I have a list of C# entities. My entity is defined as follows:

public class Item
{
    // the id of an item
    public Guid ID { get; set; }

    // if this          


        

Answer 1

Here's an alternative answer that builds a tree from the input and then traverses it in order. This traversal gives you sorted output.

class FlattenTree
{
    // map each item to its children
    ILookup mapping;

    public FlattenTree(IEnumerable list)
    {
        var itemLookup = list.ToDictionary(item => item.ID);
        mapping = list.Where(i => i.ParentID.HasValue)
                      .ToLookup(i => itemLookup[i.ParentID.Value]);
    }

    IEnumerable YieldItemAndChildren(Item node)
    {
        yield return node;
        foreach (var child in mapping[node].OrderBy(i => i.CreateDate))
            foreach (var grandchild in YieldItemAndChildren(child))
                yield return grandchild;
    }

    public IEnumerable Sort()
    {
        return from grouping in mapping
               let item = grouping.Key
               where item.ParentID == null
               orderby item.CreateDate
               from child in YieldItemAndChildren(item)
               select child;
    }
}

Invoke it like this:

var sorted = new FlattenTree(random).Sort();