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refactor: pivot is randomly chosen (#6643)

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As described in #6095, this reduces the chances to observe a O(n^2) complexity.

Here, `collection.pop(pivot_index)` is avoided for performance reasons.

Fixes: #6095
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Jérome Eertmans 2022-10-05 12:32:07 +02:00 committed by GitHub
parent 087a3a8d53
commit 8cce0d463a
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1 changed files with 10 additions and 2 deletions
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sorts/quick_sort.py
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@ -9,6 +9,8 @@ python3 quick_sort.py
""" """
from __future__ import annotations from __future__ import annotations
from random import randrange
def quick_sort(collection: list) -> list: def quick_sort(collection: list) -> list:
"""A pure Python implementation of quick sort algorithm """A pure Python implementation of quick sort algorithm
@ -26,11 +28,17 @@ def quick_sort(collection: list) -> list:
""" """
if len(collection) < 2: if len(collection) < 2:
return collection return collection
pivot = collection.pop() # Use the last element as the first pivot pivot_index = randrange(len(collection)) # Use random element as pivot
pivot = collection[pivot_index]
greater: list[int] = [] # All elements greater than pivot greater: list[int] = [] # All elements greater than pivot
lesser: list[int] = [] # All elements less than or equal to pivot lesser: list[int] = [] # All elements less than or equal to pivot
for element in collection:
for element in collection[:pivot_index]:
(greater if element > pivot else lesser).append(element) (greater if element > pivot else lesser).append(element)
for element in collection[pivot_index + 1 :]:
(greater if element > pivot else lesser).append(element)
return quick_sort(lesser) + [pivot] + quick_sort(greater) return quick_sort(lesser) + [pivot] + quick_sort(greater)