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Python Program for QuickSort

  • Difficulty Level : Medium
  • Last Updated : 22 Nov, 2021

Like Merge Sort, QuickSort is a Divide and Conquer algorithm. It picks an element as pivot and partitions the given array around the picked pivot. There are many different versions of quickSort that pick pivot in different ways. 

  1. Always pick first element as pivot.
  2. Always pick last element as pivot (implemented below)
  3. Pick a random element as pivot.
  4. Pick median as pivot.

The key process in quickSort is partition(). Target of partitions is, given an array and an element x of array as pivot, put x at its correct position in sorted array and put all smaller elements (smaller than x) before x, and put all greater elements (greater than x) after x. All this should be done in linear time. 
Pseudo Code for recursive QuickSort function : 

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/* low  --> Starting index,  high  --> Ending index */
quickSort(arr[], low, high)
    if (low < high)
        /* pi is partitioning index, arr[p] is now
           at right place */
        pi = partition(arr, low, high);

        quickSort(arr, low, pi - 1);  // Before pi
        quickSort(arr, pi + 1, high); // After pi


# Python program for implementation of Quicksort Sort
# This function takes last element as pivot, places
# the pivot element at its correct position in sorted
# array, and places all smaller (smaller than pivot)
# to left of pivot and all greater elements to right
# of pivot
def partition(arr, low, high):
    i = (low-1)         # index of smaller element
    pivot = arr[high]     # pivot
    for j in range(low, high):
        # If current element is smaller than or
        # equal to pivot
        if arr[j] <= pivot:
            # increment index of smaller element
            i = i+1
            arr[i], arr[j] = arr[j], arr[i]
    arr[i+1], arr[high] = arr[high], arr[i+1]
    return (i+1)
# The main function that implements QuickSort
# arr[] --> Array to be sorted,
# low  --> Starting index,
# high  --> Ending index
# Function to do Quick sort
def quickSort(arr, low, high):
    if len(arr) == 1:
        return arr
    if low < high:
        # pi is partitioning index, arr[p] is now
        # at right place
        pi = partition(arr, low, high)
        # Separately sort elements before
        # partition and after partition
        quickSort(arr, low, pi-1)
        quickSort(arr, pi+1, high)
# Driver code to test above
arr = [10, 7, 8, 9, 1, 5]
n = len(arr)
quickSort(arr, 0, n-1)
print("Sorted array is:")
for i in range(n):
    print("%d" % arr[i])
# This code is contributed by Mohit Kumra
#This code in improved by

Please refer complete article on QuickSort for more details!

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