Question

I have a simple tree structure represented like this:

Node
{
   int Id;
   int ParentId;
   int Name;
}

Using LINQ with a List<Node>, how can I select all nodes that doesn’t have a parent in the same list?

Example:

Id  ParentId
1   0
2   1
3   1
4   2
5   4
8   6

In the above table, Nodes with Id 1 and 8 doesn’t have a parent in the set.

Answer 1

var res = nodes.Except(from node in nodes
                       join pnode in nodes
                         on node.ParentId equals pnode.Id
                     select node);

or:

var res = from node in nodes
          join pnode in nodes
          on node.ParentId equals pnode.Id into jnodes
          from jnode in jnodes.DefaultIfEmpty()
          where jnode == null
          select node;

or:

var res = nodes.Where(np => !nodes.Any(n => np.ParentId == n.Id));