QuickSort using Random Pivoting

In this article, we will discuss how to implement QuickSort using random pivoting. In QuickSort we first partition the array in place such that all elements to the left of the pivot element are smaller, while all elements to the right of the pivot are greater that the pivot. Then we recursively call the same procedure for left and right subarrays. 
Unlike merge sort, we don’t need to merge the two sorted arrays. Thus Quicksort requires lesser auxiliary space than Merge Sort, which is why it is often preferred to Merge Sort. Using a randomly generated pivot we can further improve the time complexity of QuickSort.
We have discussed at two popular methods for partitioning the arrays-Hoare’s vs Lomuto partition scheme 
It is advised that the reader has read that article or knows how to implement the QuickSort using either of the two partition schemes.

Algorithm for random pivoting using Lomuto Partitioning

partition(arr[], lo, hi) 
    pivot = arr[hi]
    i = lo     // place for swapping
    for j := lo to hi – 1 do
        if arr[j] <= pivot then
            swap arr[i] with arr[j]
            i = i + 1
    swap arr[i] with arr[hi]
    return i

partition_r(arr[], lo, hi)
    r = Random Number from lo to hi
    Swap arr[r] and arr[hi]
    return partition(arr, lo, hi)

quicksort(arr[], lo, hi)
    if lo < hi
        p = partition_r(arr, lo, hi)
        quicksort(arr, lo , p-1)
        quicksort(arr, p+1, hi)

Implementation using Lomuto Partitioning:

C++

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// C++ implementation QuickSort
// using Lomuto's partition Scheme.
#include <cstdlib>
#include <iostream>
using namespace std;
 
// 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
int partition(int arr[], int low, int high)
{
    // pivot
    int pivot = arr[high];
   
    // Index of smaller element
    int i = (low - 1);
 
    for (int j = low; j <= high - 1; j++)
    {
        // If current element is smaller
        // than or equal to pivot
        if (arr[j] <= pivot) {
 
            // increment index of
            // smaller element
            i++;
            swap(arr[i], arr[j]);
        }
    }
    swap(arr[i + 1], arr[high]);
    return (i + 1);
}
 
// Generates Random Pivot, swaps pivot with
// end element and calls the partition function
int partition_r(int arr[], int low, int high)
{
    // Generate a random number in between
    // low .. high
    srand(time(NULL));
    int random = low + rand() % (high - low);
 
    // Swap A[random] with A[high]
    swap(arr[random], arr[high]);
 
    return partition(arr, low, high);
}
 
/* The main function that implements
QuickSort
arr[] --> Array to be sorted,
low --> Starting index,
high --> Ending index */
void quickSort(int arr[], int low, int high)
{
    if (low < high) {
 
        /* pi is partitioning index,
        arr[p] is now
        at right place */
        int pi = partition_r(arr, low, high);
 
        // Separately sort elements before
        // partition and after partition
        quickSort(arr, low, pi - 1);
        quickSort(arr, pi + 1, high);
    }
}
 
/* Function to print an array */
void printArray(int arr[], int size)
{
    int i;
    for (i = 0; i < size; i++)
        printf("%d ", arr[i]);
    printf("\n");
}
 
// Driver Code
int main()
{
    int arr[] = { 10, 7, 8, 9, 1, 5 };
    int n = sizeof(arr) / sizeof(arr[0]);
    quickSort(arr, 0, n - 1);
    printf("Sorted array: \n");
    printArray(arr, n);
    return 0;
}

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Java

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// Java program to illustrate
// Randomised Quick Sort
import java.util.*;
 
class RandomizedQsort
{    
    // This Function helps in calculating
    // random numbers between low(inclusive)
    // and high(inclusive)
    static void random(int arr[],int low,int high)
    {
     
        Random rand= new Random();
        int pivot = rand.nextInt(high-low)+low;
         
        int temp1=arr[pivot]; 
        arr[pivot]=arr[high];
        arr[high]=temp1;
    }
     
    /* 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 */
    static int partition(int arr[], int low, int high)
    {
        // pivot is choosen randomly
        random(arr,low,high);
        int pivot = arr[high];
     
 
        int i = (low-1); // index of smaller element
        for (int j = low; j < high; j++)
        {
            // If current element is smaller than or
            // equal to pivot
            if (arr[j] < pivot)
            {
                i++;
 
                // swap arr[i] and arr[j]
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }
 
        // swap arr[i+1] and arr[high] (or pivot)
        int temp = arr[i+1];
        arr[i+1] = arr[high];
        arr[high] = temp;
 
        return i+1;
    }
 
 
    /* The main function that implements QuickSort()
    arr[] --> Array to be sorted,
    low --> Starting index,
    high --> Ending index */
    static void sort(int arr[], int low, int high)
    {
        if (low < high)
        {
            /* pi is partitioning index, arr[pi] is
            now at right place */
            int pi = partition(arr, low, high);
 
            // Recursively sort elements before
            // partition and after partition
            sort(arr, low, pi-1);
            sort(arr, pi+1, high);
        }
    }
 
    /* A utility function to print array of size n */
    static void printArray(int arr[])
    {
        int n = arr.length;
        for (int i = 0; i < n; ++i)
            System.out.print(arr[i]+" ");
        System.out.println();
    }
 
    // Driver Code
    public static void main(String args[])
    {
        int arr[] = {10, 7, 8, 9, 1, 5};
        int n = arr.length;
 
        sort(arr, 0, n-1);
 
        System.out.println("Sorted array");
        printArray(arr);
    }
}
 
// This code is contributed by Ritika Gupta.

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Python3

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# Python implementation QuickSort using
# Lomuto's partition Scheme.
import random
 
'''
The function which implements QuickSort.
arr :- array to be sorted.
start :- starting index of the array.
stop :- ending index of the array.
'''
def quicksort(arr, start , stop):
    if(start < stop):
         
        # pivotindex is the index where
        # the pivot lies in the array
        pivotindex = partitionrand(arr,\
                             start, stop)
         
        # At this stage the array is
        # partially sorted around the pivot.
        # Separately sorting the
        # left half of the array and the
        # right half of the array.
        quicksort(arr , start , pivotindex-1)
        quicksort(arr, pivotindex + 1, stop)
 
# This function generates random pivot,
# swaps the first element with the pivot
# and calls the partition function.
def partitionrand(arr , start, stop):
 
    # Generating a random number between the
    # starting index of the array and the
    # ending index of the array.
    randpivot = random.randrange(start, stop)
 
    # Swapping the starting element of
    # the array and the pivot
    arr[start], arr[randpivot] = \
        arr[randpivot], arr[start]
    return partition(arr, start, stop)
 
'''
This function takes the first element as pivot,
places the pivot element at the correct position
in the sorted array. All the elements are re-arranged
according to the pivot, the elements smaller than the
pivot is places on the left and the elements
greater than the pivot is placed to the right of pivot.
'''
def partition(arr,start,stop):
    pivot = start # pivot
     
    # a variable to memorize where the
    i = start + 1
     
    # partition in the array starts from.
    for j in range(start + 1, stop + 1):
         
        # if the current element is smaller
        # or equal to pivot, shift it to the
        # left side of the partition.
        if arr[j] <= arr[pivot]:
            arr[i] , arr[j] = arr[j] , arr[i]
            i = i + 1
    arr[pivot] , arr[i - 1] =\
            arr[i - 1] , arr[pivot]
    pivot = i - 1
    return (pivot)
 
# Driver Code
if __name__ == "__main__":
    array = [10, 7, 8, 9, 1, 5]
    quicksort(array, 0, len(array) - 1)
    print(array)
 
# This code is contributed by soumyasaurav

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Output

Sorted array: 
1 5 7 8 9 10 

Algorithm for random pivoting using Hoare Partitioning



partition(arr[], lo, hi)
   pivot = arr[lo]
   i = lo - 1  // Initialize left index
   j = hi + 1  // Initialize right index

   // Find a value in left side greater
   // than pivot
   do
      i = i + 1
   while arr[i]  pivot

   if i >= j then 
      return j

   swap arr[i] with arr[j]
   
partition_r(arr[], lo, hi)
    r = Random number from lo to hi
    Swap arr[r] and arr[lo]
    return partition(arr, lo, hi)

quicksort(arr[], lo, hi)
    if lo < hi
        p = partition_r(arr, lo, hi)
        quicksort(arr, lo, p)
        quicksort(arr, p+1, hi)

Implementation using Hoare’s Partitioning:

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// C++ implementation of QuickSort
// using Hoare's partition scheme
 
#include <cstdlib>
#include <iostream>
using namespace std;
 
// 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
int partition(int arr[], int low, int high)
{
    int pivot = arr[low];
    int i = low - 1, j = high + 1;
 
    while (true) {
 
        // Find leftmost element greater than
        // or equal to pivot
        do {
            i++;
        } while (arr[i] < pivot);
 
        // Find rightmost element smaller than
        // or equal to pivot
        do {
            j--;
        } while (arr[j] > pivot);
 
        // If two pointers met
        if (i >= j)
            return j;
 
        swap(arr[i], arr[j]);
    }
}
 
// Generates Random Pivot, swaps pivot with
// end element and calls the partition function
// In Hoare partition the low element is selected
// as first pivot
int partition_r(int arr[], int low, int high)
{
    // Generate a random number in between
    // low .. high
    srand(time(NULL));
    int random = low + rand() % (high - low);
 
    // Swap A[random] with A[high]
    swap(arr[random], arr[low]);
 
    return partition(arr, low, high);
}
 
// The main function that implements QuickSort
// arr[] --> Array to be sorted,
// low  --> Starting index,
// high  --> Ending index
void quickSort(int arr[], int low, int high)
{
    if (low < high) {
        // pi is partitioning index,
        // arr[p] is now at right place
        int pi = partition_r(arr, low, high);
 
        // Separately sort elements before
        // partition and after partition
        quickSort(arr, low, pi);
        quickSort(arr, pi + 1, high);
    }
}
 
// Function to print an array
void printArray(int arr[], int n)
{
    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
    printf("\n");
}
 
// Driver Code
int main()
{
    int arr[] = { 10, 7, 8, 9, 1, 5 };
    int n = sizeof(arr) / sizeof(arr[0]);
    quickSort(arr, 0, n - 1);
    printf("Sorted array: \n");
    printArray(arr, n);
    return 0;
}

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Python3

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# Python implementation QuickSort using
# Hoare's partition Scheme.
 
import random
 
'''
The function which implements randomised
QuickSort, using Haore's partition scheme.
arr :- array to be sorted.
start :- starting index of the array.
stop :- ending index of the array.
'''
def quicksort(arr, start, stop):
    if(start < stop):
         
        # pivotindex is the index where
        # the pivot lies in the array
        pivotindex = partitionrand(arr,\
                              start, stop)
         
        # At this stage the array is
        # partially sorted around the pivot.
        # separately sorting the left half of
        # the array and the right
        # half of the array.
        quicksort(arr , start , pivotindex)
        quicksort(arr, pivotindex + 1, stop)
 
# This function generates random pivot,
# swaps the first element with the pivot
# and calls the partition function.
def partitionrand(arr , start, stop):
 
    # Generating a random number between
    # the starting index of the array and
    # the ending index of the array.
    randpivot = random.randrange(start, stop)
 
    # Swapping the starting element of
    # the array and the pivot
    arr[start], arr[randpivot] =\
        arr[randpivot], arr[start]
    return partition(arr, start, stop)
 
'''
This function takes the first element
as pivot, places the pivot element at
the correct position in the sorted array.
All the elements are re-arranged according
to the pivot, the elements smaller than
the pivot is places on the left and
the elements greater than the pivot is
placed to the right of pivot.
'''
def partition(arr,start,stop):
    pivot = start # pivot
    i = start - 1
    j = stop + 1
    while True:
        while True:
            i = i + 1
            if arr[i] >= arr[pivot]:
                break
        while True:
            j = j - 1
            if arr[j] <= arr[pivot]:
                break
        if i >= j:
            return j
        arr[i] , arr[j] = arr[j] , arr[i]
 
# Driver Code
if __name__ == "__main__":
    array = [10, 7, 8, 9, 1, 5]
    quicksort(array, 0, len(array) - 1)
    print(array)
 
# This code is contributed by soumyasaurav

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Output

Sorted array: 
1 5 7 8 9 10 

Analysis of Randomized Quick Sort

Notes 

  • Using random pivoting we improve the expected or average time complexity to O (N log N). The Worst-Case complexity is still O ( N^2 ).

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