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Re: Sort one sequence by O(n) in time and O(1) in space

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In article <ae372652-0f1c-4d79-82db-a522eb592579@googlegroups.com>,
 Wesley <nispray@gmail.com> wrote:

> Hi guys,
>    Here is one question related to algorithm.
> Details here:
> 
> here is input sequence like a1,a2,...,an,b1,b2,...,bn ，the ax and bx always 
> exist in pair. So, now, how to change the sequence to a1,b1,...,an,bn, with 
> time complexity as O(n) and space as O(1).

Is this a homework problem?

It sounds like what you want to do is split the list into two halves, then shuffle them.  Using
itertools let's you do these operations without making copies of the data.  

Something like:

--------------------------------------------------
from itertools import islice, izip

def cut_and_shuffle(input):
    n = len(input)
    if n % 2:
        raise ValueError("input length must be even")
    split = n/2
    print "split =", split
    first_half = islice(input, 0, split)
    second_half = islice(input, split, n)
    for a, b in izip(first_half, second_half):
        yield a
        yield b

l = ['a1', 'a2', 'a3', 'a4', 'b1', 'b2', 'b3', 'b4']
print list(cut_and_shuffle(l))
--------------------------------------------------

$ python shuffle.py
split = 4
['a1', 'b1', 'a2', 'b2', 'a3', 'b3', 'a4', 'b4']

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Sort one sequence by O(n) in time and O(1) in space Wesley <nispray@gmail.com> - 2014-02-09 04:13 -0800
  Re: Sort one sequence by O(n) in time and O(1) in space Asaf Las <roegltd@gmail.com> - 2014-02-09 04:29 -0800
  Re: Sort one sequence by O(n) in time and O(1) in space Chris Angelico <rosuav@gmail.com> - 2014-02-09 23:39 +1100
    Re: Sort one sequence by O(n) in time and O(1) in space Wesley <nispray@gmail.com> - 2014-02-09 18:09 -0800
      Re: Sort one sequence by O(n) in time and O(1) in space Dave Angel <davea@davea.name> - 2014-02-09 22:00 -0500
  Re: Sort one sequence by O(n) in time and O(1) in space Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2014-02-09 13:41 +0000
  Re: Sort one sequence by O(n) in time and O(1) in space Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2014-02-09 14:30 +0000
  Re: Sort one sequence by O(n) in time and O(1) in space Ni Wesley <nispray@gmail.com> - 2014-02-09 22:40 +0800
  Re: Sort one sequence by O(n) in time and O(1) in space Roy Smith <roy@panix.com> - 2014-02-09 10:05 -0500
    Re: Sort one sequence by O(n) in time and O(1) in space Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-09 23:14 +0000
      Re: Sort one sequence by O(n) in time and O(1) in space Roy Smith <roy@panix.com> - 2014-02-09 19:15 -0500
      Re: Sort one sequence by O(n) in time and O(1) in space Wesley <nispray@gmail.com> - 2014-02-09 18:26 -0800
        Re: Sort one sequence by O(n) in time and O(1) in space Sturla Molden <sturla.molden@gmail.com> - 2014-02-10 10:20 +0000
          Re: Sort one sequence by O(n) in time and O(1) in space Steven D'Aprano <steve@pearwood.info> - 2014-02-11 05:20 +0000
            Re: Sort one sequence by O(n) in time and O(1) in space Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-12 11:56 +1300
          Re: Sort one sequence by O(n) in time and O(1) in space Tim Daneliuk <tundra@tundraware.com> - 2014-02-10 23:37 -0600
          Re: Sort one sequence by O(n) in time and O(1) in space Tim Daneliuk <tundra@tundraware.com> - 2014-02-10 23:37 -0600
        Re: Sort one sequence by O(n) in time and O(1) in space Chris Angelico <rosuav@gmail.com> - 2014-02-10 21:37 +1100
        Re: Sort one sequence by O(n) in time and O(1) in space Sturla Molden <sturla.molden@gmail.com> - 2014-02-10 15:03 +0000
        Re: Sort one sequence by O(n) in time and O(1) in space Chris Angelico <rosuav@gmail.com> - 2014-02-11 02:23 +1100
        Re: Sort one sequence by O(n) in time and O(1) in space Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2014-02-10 15:45 +0000
        Re: Sort one sequence by O(n) in time and O(1) in space Chris Angelico <rosuav@gmail.com> - 2014-02-11 02:52 +1100
          Re: Sort one sequence by O(n) in time and O(1) in space Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-11 13:19 +1300
        Re: Sort one sequence by O(n) in time and O(1) in space Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2014-02-10 16:41 +0000
  Re: Sort one sequence by O(n) in time and O(1) in space Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2014-02-09 15:48 +0000
    Re: Sort one sequence by O(n) in time and O(1) in space Wesley <nispray@gmail.com> - 2014-02-09 18:07 -0800
  Re: Sort one sequence by O(n) in time and O(1) in space Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-09 21:31 +0000

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