// Move the nth disk of the first tower
        public void MoveDiskFromFirstTowerToLast(int n)
        {
            if (first == null || n == 0)
            {
                return;
            }

            // Pop the disks that's smaller than n in the 1st stack
            List <int> firstDisks = PopDisksFromtheStackAboveDiskN(first, n);

            // Get disk N from the 1st stack
            int diskN = first.pop();

            // Put the disks back into the 1st stack
            PushDisksBackTotheStack(first, firstDisks);

            if (second != null)
            {
                // Insert diskN to the 2nd stack
                List <int> secDisks = InsertDiskIntoAStack(second, diskN, false);

                // Put the disks back into the 2nd stack
                PushDisksBackTotheStack(second, secDisks);
            }

            if (third == null)
            {
                third = new StackOfDisks();
            }

            // Insert diskN to the 3rd stack
            List <int> thdDisks = InsertDiskIntoAStack(third, diskN, true);

            // Put the disks back into the 3rd stack
            PushDisksBackTotheStack(third, thdDisks);
        }
        public List <int> InsertDiskIntoAStack(StackOfDisks s, int diamterOfDisk, bool putdown)
        {
            List <int> disks = new List <int>();

            while (true)
            {
                int d = s.pop();
                if (d < diamterOfDisk)
                {
                    disks.Add(d);
                }
                else
                {
                    s.push(d); // Put the bigger disk back to stack
                    if (putdown)
                    {
                        s.push(diamterOfDisk); // put the diskN to the stack
                    }
                    break;
                }
            }
            return(disks);
        }