original question is posted in http://www.spoj.pl/problems/CODESPTC/ Here is an algorithm for shuffling N cards:
1) The cards are divided into K equal piles, where K is a factor of N.
2) The bottom N / K cards belong to pile 1 in the same order (so the bottom card of the .initial pile is the bottom card of pile 1).
3) The next N / K cards from the bottom belong to pile 2, and so on.
4) Now the top card of the shuffled pile is the top card of pile 1. The next card is the top card of pile 2,..., the Kth card of the shuffled pile is the top card of pile K. Then (K + 1)th card is the card which is now at the top of pile 1, the (K + 2)nd is the card which is now at the top of pile 2 and so on.
For example, if N = 6 and K = 3, the order of a deck of cards "ABCDEF" (top to bottom) when shuffled once would change to "ECAFDB".
Given N and K, what is the least number of shuffles needed after which the pile is restored to its original order?
What i am looking for is mathematical equation to get the answer. i simulated and observed that a card x will have the new position as given by x = (x%k)*k + k - 1 - x/k . We start with x=0. In every iteration we get new value of x. once x is 0 again, we are in the original configuration. so the answer is the number of iterations. Is there better and faster way to do it?