Problem

Given a string, write a function that uses recursion to output a list of all the possible permutations of that string.

For example, given s=’abc’ the function should return [‘abc’, ‘acb’, ‘bac’, ‘bca’, ‘cab’, ‘cba’]

Note: If a character is repeated, treat each occurence as distinct, for example an input of ‘xxx’ would return a list with 6 “versions” of ‘xxx’

Solution

Let’s think about what the steps we need to take here are:

def permute(s):
    output = []
    
    if len(s) <= 1:
        output=[s]
    else:
        for i, let in enumerate(s):
            for perm in permute(s[:i]+s[i+1:]):
                output+=[let+perm]
    return output

permute('abc')

Test Your Solution

"""
RUN THIS CELL TO TEST YOUR SOLUTION.
"""

from nose.tools import assert_equal

class TestPerm(object):
    
    def test(self,solution):
        
        assert_equal(sorted(solution('abc')),sorted(['abc', 'acb', 'bac', 'bca', 'cab', 'cba']))
        assert_equal(sorted(solution('dog')),sorted(['dog', 'dgo', 'odg', 'ogd', 'gdo', 'god']))
        
        print ('All test cases passed.')
        


# Run Tests
t = TestPerm()
t.test(permute)

All test cases passed.

Recommended: Introduction to Recursion

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Hassan Raza