Last active
March 28, 2019 21:43
-
-
Save lopespm/81a336871ce6074f63f3cad349c3a95d to your computer and use it in GitHub Desktop.
Dutch national flag problem: four colors - in Python
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
# (Variant for exercise 5.1 on EPI (Elements of Programming Interviews)) | |
# The rationale behind it is to squeeze the forth color (middle right color) in between the middle left and right sub-arrays. It defines the colors as the algorithm progresses. | |
# It has a O(n) time complexity and O(1) space complexity | |
from typing import List | |
def dutch_variant_four_colors(array: List[int]) -> List[int]: | |
left = array[0] | |
mid_left = None | |
right = None | |
left_i = 0 | |
mid_left_i = 0 | |
mid_right_i = len(array) | |
right_i = len(array) | |
while mid_left_i < right_i and mid_left_i < mid_right_i: | |
if (array[mid_left_i] == left): | |
array[mid_left_i], array[left_i] = array[left_i], array[mid_left_i] | |
mid_left_i += 1 | |
left_i += 1 | |
elif (right is None or array[mid_left_i] == right): | |
right_i -= 1 | |
mid_right_i = right_i | |
array[mid_left_i], array[right_i] = array[right_i], array[mid_left_i] | |
right = array[right_i] | |
else: # it is a mid value | |
if (mid_left is None): | |
mid_left = array[mid_left_i] | |
if (array[mid_left_i] == mid_left): | |
mid_left_i += 1 | |
else: | |
mid_right_i -= 1 | |
array[mid_left_i], array[mid_right_i] = array[mid_right_i], array[mid_left_i] | |
return array | |
# Sample usages | |
print(dutch_variant_four_colors([1,1,3,3,4,3,2,2])) | |
print(dutch_variant_four_colors([1,2,3,4,2,3,1,3])) | |
print(dutch_variant_four_colors([1,2,3,4,4])) | |
print(dutch_variant_four_colors([0,1,2,5,5,2,2,0])) | |
print(dutch_variant_four_colors([1,0,3,0,5,5])) | |
print(dutch_variant_four_colors([1,2,3,2,5,1,1,3])) | |
print(dutch_variant_four_colors([5,1,2,3,2,1,1,3])) | |
print(dutch_variant_four_colors([3,2,5,1])) | |
print(dutch_variant_four_colors([3,2,1,5])) | |
print(dutch_variant_four_colors([3,2,1,3,5,2,2,1,2,3,5,3,2,1,3,5,3,3,2,2,2,5,5,5,3,3,2,5,3,1,2,3,2,1,3,2,1,1,2,3,2,3,2,1,2,3,2,1,2,3,2,2,2,2,2,3,3,3,1,2,2,1,1,2,3])) |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment