bool DFS (currentState) =
    {
          if (myHashSet.Contains(currentState))
          {
               return;
          }
          else
          {
                myHashSet.Add(currentState);
          }
          if (IsSolution(currentState) return true;
          else
          {
              for (var nextState in GetNextStates(currentState))
                   if (DFS(nextState)) return true;
          }
          return false;
     }

bool DFS (currentState, maxDepth) =
    {
          if (maxDepth = 0) return false;
          if (myHashSet.Contains(currentState))
          {
               return;
          }
          else
          {
                myHashSet.Add(currentState);
          }
          if (IsSolution(currentState) return true;
          else
          {
              for (var nextState in GetNextStates(currentState))
                   if (DFS(nextState, maxDepth - 1)) return true;
          }
          return false;
     }

A - B - C - D - E - A
                   |
                   F - G (Goal)

bool DFS (currentState, maxDepth) =
{
      if (maxDepth = 0) return false;
      if (myHashSet.Contains(currentState))
      {
           return;
      }
      else
      {
            myHashSet.Add(currentState);
      }
      if (IsSolution(currentState) return true;
      else
      {
          for (var nextState in GetNextStates(currentState))
               if (DFS(nextState, maxDepth - 1)) return true;
      }
      myHashSet.Remove(currentState); //the state is pop out from the stack
      return false;
 }