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757d4fb84f
* Added Dutch national flag sort Algorithm * Changed file name to dnf_sort.py * Added descriptive name and type hint Added descriptive name and type hint for parameter with doctest for the function dnf_sort. * Added test cases * Added doctest cases * Update sorts/dnf_sort.py * Added doctest for dutch_national_flag_sort sorts/dnf_sort.py * Update sorts/dnf_sort.py * Added doctest for the function dutch_national_flag_sort * update file as per black code formatter * Update dnf_sort.py * Update and rename dnf_sort.py to dutch_national_flag_sort.py Co-authored-by: Christian Clauss <cclauss@me.com>
101 lines
3.6 KiB
Python
101 lines
3.6 KiB
Python
"""
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A pure implementation of Dutch national flag (DNF) sort algorithm in Python.
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Dutch National Flag algorithm is an algorithm originally designed by Edsger Dijkstra.
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It is the most optimal sort for 3 unique values (eg. 0, 1, 2) in a sequence. DNF can
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sort a sequence of n size with [0 <= a[i] <= 2] at guaranteed O(n) complexity in a
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single pass.
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The flag of the Netherlands consists of three colors: white, red, and blue.
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The task is to randomly arrange balls of white, red, and blue in such a way that balls
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of the same color are placed together. DNF sorts a sequence of 0, 1, and 2's in linear
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time that does not consume any extra space. This algorithm can be implemented only on
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a sequence that contains three unique elements.
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1) Time complexity is O(n).
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2) Space complexity is O(1).
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More info on: https://en.wikipedia.org/wiki/Dutch_national_flag_problem
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For doctests run following command:
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python3 -m doctest -v dutch_national_flag_sort.py
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For manual testing run:
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python dnf_sort.py
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"""
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# Python program to sort a sequence containing only 0, 1 and 2 in a single pass.
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red = 0 # The first color of the flag.
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white = 1 # The second color of the flag.
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blue = 2 # The third color of the flag.
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colors = (red, white, blue)
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def dutch_national_flag_sort(sequence: list) -> list:
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"""
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A pure Python implementation of Dutch National Flag sort algorithm.
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:param data: 3 unique integer values (e.g., 0, 1, 2) in an sequence
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:return: The same collection in ascending order
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>>> dutch_national_flag_sort([])
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[]
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>>> dutch_national_flag_sort([0])
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[0]
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>>> dutch_national_flag_sort([2, 1, 0, 0, 1, 2])
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[0, 0, 1, 1, 2, 2]
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>>> dutch_national_flag_sort([0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1])
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[0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2]
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>>> dutch_national_flag_sort("abacab")
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Traceback (most recent call last):
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...
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ValueError: The elements inside the sequence must contains only (0, 1, 2) values
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>>> dutch_national_flag_sort("Abacab")
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Traceback (most recent call last):
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...
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ValueError: The elements inside the sequence must contains only (0, 1, 2) values
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>>> dutch_national_flag_sort([3, 2, 3, 1, 3, 0, 3])
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Traceback (most recent call last):
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...
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ValueError: The elements inside the sequence must contains only (0, 1, 2) values
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>>> dutch_national_flag_sort([-1, 2, -1, 1, -1, 0, -1])
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Traceback (most recent call last):
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...
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ValueError: The elements inside the sequence must contains only (0, 1, 2) values
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>>> dutch_national_flag_sort([1.1, 2, 1.1, 1, 1.1, 0, 1.1])
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Traceback (most recent call last):
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...
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ValueError: The elements inside the sequence must contains only (0, 1, 2) values
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"""
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if not sequence:
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return []
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if len(sequence) == 1:
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return list(sequence)
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low = 0
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high = len(sequence) - 1
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mid = 0
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while mid <= high:
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if sequence[mid] == colors[0]:
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sequence[low], sequence[mid] = sequence[mid], sequence[low]
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low += 1
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mid += 1
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elif sequence[mid] == colors[1]:
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mid += 1
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elif sequence[mid] == colors[2]:
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sequence[mid], sequence[high] = sequence[high], sequence[mid]
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high -= 1
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else:
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raise ValueError(
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f"The elements inside the sequence must contains only {colors} values"
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)
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return sequence
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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user_input = input("Enter numbers separated by commas:\n").strip()
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unsorted = [int(item.strip()) for item in user_input.split(",")]
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print(f"{dutch_national_flag_sort(unsorted)}")
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