Detect cycles in a genealogy graph during a Depth-first search

Question

I'm loading horse genealogical data recursively. For some wrong sets of data my recursion never stops... and that is because there are cycles in the data.

How can I detect those cycles to stop recurring?

I thought of while recurring maintain a hashTable with all "visited" horses. But that will find some false positives, because a horse CAN be twice in a tree.

What cannot happen is that a horse appear as a father or grandfather or great grandfather of ITSELF.

Answer 1

Pseudo code:

void ProcessTree(GenTreeNode currentNode, Stack<GenTreeNode> seen)
{
   if(seen.Contains(currentNode)) return;
   // Or, do whatever needs to be done when a cycle is detected

   ProcessHorse(currentNode.Horse); // Or whatever processing you need

   seen.Push(currentNode);

   foreach(GenTreeNode childNode in currentNode.Nodes)
   {
      ProcessTree(childNode, seen);
   }

   seen.Pop();
}

The basic idea is to keep a list of all of the nodes that we've already seen on our way down to the current node; if was get back to a node that we already went through, then you know that we've formed a cycle (and we should skip the value, or do whatever needs to be done)