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

Solution 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'


  • For each char in string
    • If char is the same as last_char, increment count
    • Else
      • Append last_char and count to compressed_string
      • last_char = char
      • count = 1
  • Append last_char and count to compressed_string
  • If the compressed string size is < string size
    • Return compressed string
  • Else
    • Return string


  • Time: O(n)
  • Space: O(n)

Complexity Note:

  • Although strings are immutable in Python, appending to strings is optimized in CPython so that it now runs in O(n) and extends the string in-place. Refer to this Stack Overflow post.


In [1]:
class CompressString(object):

    def compress(self, string):
        if string is None or not string:
            return string
        result = ''
        prev_char = string[0]
        count = 0
        for char in string:
            if char == prev_char:
                count += 1
                result += self._calc_partial_result(prev_char, count)
                prev_char = char
                count = 1
        result += self._calc_partial_result(prev_char, count)
        return result if len(result) < len(string) else string

    def _calc_partial_result(self, prev_char, count):
        return prev_char + (str(count) if count > 1 else '')

Unit Test

In [2]:
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__':
In [3]:
%run -i
Success: test_compress