First variant of Graph class.
Fix toString format.
breadth-first and depth-first search complete.
Added digraph implementation.
Added DirectedCycle class - checks if Graph has cycle. Calling toString method returns the first cycle or message "Graph has no cycles".
Implemented topological sort for digraphs.
Implemented graph structures for MST.
Fix addEdge.
Prim algorithm implemented.
Kruskal algorithm implemented.
Structures for MIN Path search.
Dijkstra algorithm implementation