This notebook was prepared by Donne Martin. Source and license info is on GitHub.

Challenge Notebook

Problem: Compress a string such that 'AAABCCDDDD' becomes 'A3BC2D4'. Only compress the string if it saves space.


  • Can we assume the string is ASCII?
    • Yes
    • Note: Unicode strings could require special handling depending on your language
  • Is this case sensitive?
    • Yes
  • Can we use additional data structures?
    • Yes
  • Can we assume this fits in memory?
    • Yes

Test Cases

  • None -> None
  • '' -> ''
  • 'AABBCC' -> 'AABBCC'
  • 'AAABCCDDDD' -> 'A3BC2D4'


Refer to the Solution Notebook. If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start.


In [ ]:
class CompressString(object):

    def compress(self, string):
        # TODO: Implement me

Unit Test

The following unit test is expected to fail until you solve the challenge.

In [ ]:
# %load
import unittest

class TestCompress(unittest.TestCase):

    def test_compress(self, func):
        self.assertEqual(func(None), None)
        self.assertEqual(func(''), '')
        self.assertEqual(func('AABBCC'), 'AABBCC')
        self.assertEqual(func('AAABCCDDDDE'), 'A3BC2D4E')
        self.assertEqual(func('BAAACCDDDD'), 'BA3C2D4')
        self.assertEqual(func('AAABAACCDDDD'), 'A3BA2C2D4')
        print('Success: test_compress')

def main():
    test = TestCompress()
    compress_string = CompressString()

if __name__ == '__main__':

Solution Notebook

Review the Solution Notebook for a discussion on algorithms and code solutions.