From: Shaw xx <lists@...>
Date: 2013-04-13T00:03:38+09:00
Subject: Looking for a more elegant solution.

I'm new to ruby and working on such a problem:

There n numbered letters and n numbered envelops.  The letter x can't be
put into the envelop x. What I want is to print out all the possible
cases.

[letter(x1), letter(x2), ... , letter(xn)]
      |            |                  |
 envelop1, envelop2, ... ,  envelop n

The index of Array + 1 ---> the number of the envelop

The element of Array ---> the number of the letter

Input:      n = 3.
Output:    [1, 3, 2], [2, 3, 1]

Input:     n = 4.
Output:   [2, 1, 4, 3], [2, 3, 4, 1], [2, 4, 1, 3], [3, 1, 4, 2],
          [3, 4, 1, 2], [3, 4, 2, 1], [4, 1, 2, 3], [4, 3, 1, 2],
          [4, 3, 2, 1]

Here is my code:
-----------------------
$nums = []

def f( already, n, times )
  if n > times
    $nums << already.dup
    return
  else
    1.upto(times) do |i|
      next if ((already.include? i) || n == i)
      already << i
      f( already, n+1, times )
      already.pop
    end
  end
end
------------------------

Sorry for my poor English. Thank you very much.

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