Consider the following page-reference string: 1, 2, 3, 4, 2, 1, 5, 6, 2, 1, 2, 3, 7, 6, 3, 2, 1, 2, 3, 6

How many page faults would occur for following replacement
algorithms for a memory with 4 frames? Remember that all frames
are initially empty, so your first unique pages will all cost one
fault each.
(i)  LRU replacement.
(ii)  FIFO replacement.
(iii)  Optimal replacement.