Quicksort with Python

Quick sort without additional memory (in place)

Usage:

array = [97, 200, 100, 101, 211, 107]
quicksort(array)
print(array)
# array -> [97, 100, 101, 107, 200, 211]

def partition(array, begin, end):
    pivot = begin
    for i in range(begin+1, end+1):
        if array[i] <= array[begin]:
            pivot += 1
            array[i], array[pivot] = array[pivot], array[i]
    array[pivot], array[begin] = array[begin], array[pivot]
    return pivot



def quicksort(array, begin=0, end=None):
    if end is None:
        end = len(array) - 1
    def _quicksort(array, begin, end):
        if begin >= end:
            return
        pivot = partition(array, begin, end)
        _quicksort(array, begin, pivot-1)
        _quicksort(array, pivot+1, end)
    return _quicksort(array, begin, end)

def sort(array):
    """Sort the array by using quicksort."""

    less = []
    equal = []
    greater = []

    if len(array) > 1:
        pivot = array[0]
        for x in array:
            if x < pivot:
                less.append(x)
            elif x == pivot:
                equal.append(x)
            elif x > pivot:
                greater.append(x)
        # Don't forget to return something!
        return sort(less)+equal+sort(greater)  # Just use the + operator to join lists
    # Note that you want equal ^^^^^ not pivot
    else:  # You need to handle the part at the end of the recursion - when you only have one element in your array, just return the array.
        return array

There is another concise and beautiful version

def qsort(arr):
    if len(arr) <= 1:
        return arr
    else:
        return qsort([x for x in arr[1:] if x < arr[0]])
        + [arr[0]]
        + qsort([x for x in arr[1:] if x >= arr[0]])

Let me explain the above codes for details

1. pick the first element of array arr[0] as pivot

   [arr[0]]

2. qsort those elements of array which are less than pivot with List Comprehension

   qsort([x for x in arr[1:] if x < arr[0]])

3. qsort those elements of array which are larger than pivot with List Comprehension

   qsort([x for x in arr[1:] if x >= arr[0]])