// Graph abstract class. This ADT assumes that the number
// of vertices is fixed when the graph is created.
class Graph {
public:
  // Return the number of vertices in the graph
  virtual int n() =0;

  // Return the current number of edges in the graph
  virtual int e() =0;

  // Store an edge from v1 to v2 with weight wgt
  virtual void setEdge(int v1, int v2, int wgt) =0;

  // Delete the edge going from v1 to v2
  virtual void delEdge(int v1, int v2) =0;

  // Return weight of the edge from v1 to v2.
  // Return 0 if no such edge exists.
  virtual int weight(int v1, int v2) =0;

  // Get the mark value for vertex v
  virtual int getMark(int v) =0;

  //  Set the mark value for vertex v to be val
  virtual void setMark(int v, int val) =0;

  // Return the index of the first neighbor for vertex v
  virtual int first(int v) =0;

  // Return the index of the next neighbor
  // (after v2) for vertex v1
  virtual int next(int v1, int v2) =0;
};