import unittest

class Solution:

def __init__(self):

self.stack = []

def combination(self, arr, offset, length, k, result):

if k == 0:

result.append([''.join(self.stack[:])])

return

for i in range(offset, length):

self.stack.append(arr[i])

self.combination(arr, i, length, k - 1, result)

self.stack.pop()

class TestCombination(unittest.TestCase):

def test(self):

sol = Solution()

arr = ['A', 'B', 'C']

result = []

sol.combination(arr, 0, len(arr), 2, result)

self.assertEqual(

result,

[

['AA'],

['AB'],

['AC'],

['BB'],

['BC'],

['CC'],

]

)

result = []

sol.combination(arr, 0, len(arr), 3, result)

self.assertEqual(

result,

[

['AAA'],

['AAB'],

['AAC'],

['ABB'],

['ABC'],

['ACC'],

['BBB'],

['BBC'],

['BCC'],

['CCC'],

]

)

if __name__ == '__main__':

unittest.TestCase()