Cycle detection

What if we introduce a cycle? For example, refer to the cycle shown here:

If you invoke the topsort method with this graph as an argument, it will never be completed:

scala> topsort(("dinner", "movie") :: grwork) 
java.lang.StackOverflowError 
...  

The fix is to remember that x is already seen, going to precede, so processing x again is a cycle. We just abort the execution in this case:

scala> def topsortWithCycle(g: List[(String, String)]) = { 
     |   def sort(nodes: List[String], path: List[String], visited: List[String]): 
     |   List[String] = nodes match { 
     |     case Nil => visited 
     |     case x :: xs if path.contains(x) => 
     |       throw new RuntimeException("Cycle detected") 
     |     case x :: xs => sort(xs, path, 
     |       if (visited.contains(x)) visited 
     |       else x :: sort(succSet(x, g), x :: path, visited)) 
     |   } 
     | 
     |   val (start, _) = g.unzip 
     |   val result = sort(start, List(), List()...