quicksort.py

def swap(array, first, second):
    array[first], array[second] = array[second], array[first]

def quick_sort(array):
    return _quick_sort(array, 0, len(array) - 1)

def _quick_sort(array, start, end):
    if start < end:
        pivot_index = partition(array, start, end)
        _quick_sort(array, start, pivot_index - 1)
        _quick_sort(array, pivot_index + 1, end)
        return 
    else:
        return 0
    
def partition(array, start, end):
    pivot = array[start]
    i = start + 1

    for j in range(start + 1, end + 1):
        if array[j] < pivot:
            swap(array, j, i)
            i = i + 1

    swap(array, start, i - 1)
    return i - 1

input_array = [20, 18, 99, 3, 65, 23, 55, 4]
quick_sort(input_array)
print input_array