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.¶

Constraints¶

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'

Algorithm¶

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

Complexity:

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.

Code¶

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
            else:
                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]:

%%writefile test_compress.py
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()
    test.test_compress(compress_string.compress)


if __name__ == '__main__':
    main()

Overwriting test_compress.py

In [3]:

%run -i test_compress.py

Success: test_compress