DirectedGraphAdjacencyList.PreOrderDepthFirstTraversalRecursiveIterator
DirectedGraphAdjacencyList.PreOrderDepthFirstTraversalRecursiveIterator

Update to method
DirectedGraphAdjacencyList.PreOrderDepthFirstTraversalRecursiveIterator

- Minor and clearer rewriting
- Abort traversal if root of traversal is already visited

lopezpdvn/DataStructuresAlgorithmsCSharp