private ArrayList<ArrayList<Integer>> solve(int n) {
      int solutionCount = 0;

      if (n == ironRod.size()) {
         return ironRod; // base case
      }
      int rotationCount = 0;
      boolean solved = false;
      while (!solved) {
         solved = true;
         for (int j = 1; j < n; j++) {
            if ( shareSides(ironRod.get(n), ironRod.get(j)) ) { // tests if cubes share sides

               solved = false;
               break;
            } // end if
         } // end for

         if (solved && (solve(n+1) != null)) {

            if (solutionCount++ < 5) {
               System.out.println(ironRod);
               return null;
            }
         }
         else {
            solved = false;
            ironRod.set(n, this.rotate(ironRod.get(n)));
            rotationCount += 1;
            if (rotationCount > 3)
               return null;
         } // end else

      } // end while
      return null;
   } // end solve(int n)