Building hierarchy objects from flat list of parent/child

I have a list of items in a hierarchy, and I'm attempting to parse this list out into an actual hierarchy of objects. I'm using modified pre-order tree traversal to store/iterate through this list, and so what I have is a subset of the tree, including all children, ordered by their "left" value.

For example, given the tree:

• Item A
  • Item A.1
  • Item A.2
    • Item A.2.2
• Item B
  • Item B.1
• Item C

I get the list:

• Item A, Item A.1, Item A.2, Item A.2.2, Item B, Item B.1, Item C

(This is in order of the "left" value from the modified pre-order tree setup).

What I want to do is parse this into objects that contain the actual structure of the tree, eg:

Class TreeObject {
    String Name;
    Guid ID; 
    Guid ParentID;
    List<TreeObject> Children;
}

The flat list is returned as a List of TreeObjects - and each TreeObject has properties for ID, ParentID, Left, and Right. What I'm looking for is a function:

List<TreeObject> FlatToHeirarchy(List<TreeObject> list); 

which takes the flat list in, and returns a nested list.

In other words:

List<TreeObject> flatSet = LoadTreeObjectsFromDatabase(); 
// flatSet.count == 7; flatSet(0).Children == null
List<TreeObject> nestedSet = FlatToHeirarchy(flatSet);
// nestedSet.count == 3; nestedSet(0).Children.count == 2

I'm at a loss how to do this - keeping track of parents, and being able to deal with a bigger jump (eg, Item A.2.2 -> Item B).

---

Edit: I'm looking for a non-brute-force solution here (eg, not looping several times, moving items into child nodes, until there are only the top-level parents left). I'm guessing there is an elegant method that can loop once, and just place items as needed.

Remember, they are always in a hierarchal order (since I'm using MPTT), so a given item is going to always be a child or sibling of the previous item, or at least share a parent with the previous item. It is never going to come somewhere else in the tree.

Here's the function I ended up writing. I'm using MPTT to store objects, so the list is in order of the 'left' value, which basically means the parent always comes before any given item in the list. In other words, the item referenced by item.ParentID has always already been added (except in the case of top-level or root nodes).

public class TreeObject
{
    public int Id { get; set; }
    public int ParentId { get; set; }
    public string Name { get; set; }
    public IList<TreeObject> Children { get; set; } = new List<TreeObject>();
}

public IEnumerable<TreeObject> FlatToHierarchy(List<TreeObject> list)
{
    // hashtable lookup that allows us to grab references to containers based on id
    var lookup = new Dictionary<int, TreeObject>();
    // actual nested collection to return
    var nested = new List<TreeObject>();

    foreach (TreeObject item in list)
    {
        if (lookup.ContainsKey(item.ParentId))
        {
            // add to the parent's child list 
            lookup[item.ParentId].Children.Add(item);
        }
        else
        {
            // no parent added yet (or this is the first time)
            nested.Add(item);
        }
        lookup.Add(item.Id, item);
    }

    return nested;
}

and a simple test (that works in LinqPad):

void Main()
{
    var list = new List<TreeObject>() {
        new TreeObject() { Id = 1, ParentId = 0, Name = "A" },
        new TreeObject() { Id = 2, ParentId = 1, Name = "A.1" },
        new TreeObject() { Id = 3, ParentId = 1, Name = "A.2" },
        new TreeObject() { Id = 4, ParentId = 3, Name = "A.2.i" },
        new TreeObject() { Id = 5, ParentId = 3, Name = "A.2.ii" }
    };

    FlatToHierarchy(list).Dump();
}

Results:

Since I'm updating this 5 years later, here's a recursive LINQ version:

public IList<TreeObject> FlatToHierarchy(IEnumerable<TreeObject> list, int parentId = 0) {
    return (from i in list 
            where i.ParentId == parentId 
            select new TreeObject {
                Id = i.Id, 
                ParentId = i.ParentId,
                Name = i.Name,
                Children = FlatToHierarchy(list, i.Id)
            }).ToList();
}