Building hierarchy objects from flat list of parent/child

Question

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/iter

Answer 1

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 Children { get; set; } = new List();
}

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

    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() {
        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 FlatToHierarchy(IEnumerable 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();
}