How to Fix Quick Sort Errors

In this tutorial, you'll learn about How to Fix Quick Sort Errors. We cover key concepts, practical examples, and best practices.

Fix quick sort errors when pivot choice causes O(n^2) on sorted arrays or partition logic incorrect.

Quick Fix

Wrong

def qs(a):
    if len(a)<=1: return a
    p=a[0]; l=[x for x in a[1:] if x<=p]; r=[x for x in a[1:] if x>p]
    return qs(l)+[p]+qs(r)

O(n^2) on already sorted array. Extra O(n) space per recursion level.

Right

import random
def qs(a):
    if len(a)<=1: return a
    p=random.choice(a)
    l=[x for x in a if x<p]; m=[x for x in a if x==p]; r=[x for x in a if x>p]
    return qs(l)+m+qs(r)

[3,6,8,10,1,2,1] -> [1,1,2,3,6,8,10]. O(n log n) expected.

Prevention

Randomize pivot selection. Handle duplicates with equal-to-pivot partition. O(n log n) expected.

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