interesting exercise

[Gabriel Genellina]

En Tue, 08 May 2007 00:45:52 -0300, Michael Tobis <mtobis at gmail.com>  
escribió:

> I want a list of all ordered permutations of a given length of a set
> of tokens. Each token is a single character, and for convenience, they
> are passed as a string in ascending ASCII order.

This is what I come, no tricks, no special optimizations, just plain  
Python (including your constraints):

def permute(values, n):

   def _permute(values, n):
     if n==1:
       for letter in values:
         yield [letter]
     else:
       for letter in values:
         for others in _permute(values, n-1):
           yield [letter]+others

   if not sorted(values): raise ValueError("unsorted values")
   if len(set(values))!=len(values): raise ValueError("duplicate values")
   return list(''.join(item) for item in _permute(values, n))

-- 
Gabriel Genellina