python-trials/math/permutations.py

#  Copyright (C) 2020, 2019 Girish M
#  This program is free software; you can redistribute it and/or modify
#  it under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 3 of the License, or
#  (at your option) any later version.
#  
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
#  
#  You should have received a copy of the GNU General Public License
#  along with this program; if not, write to the Free Software
#  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
#  MA 02110-1301, USA.
# 
l = ['a', 'b']

def perm(l, start, end):
    ''' generate all the permutations of list[start] to list[end] '''
    if start==end:
        for j in l:
            print j
        print "    "
    else:
        ''' list[start] to list[end] has more than one permutation, generate these recursively'''
        for j in l[start:end]:
            l[start], j = j, l[start]
            perm(l, start+1, end)
            l[start], j = j, l[start] #restore original list

perm(l, 0, 2)