QuickSort – Data Structure and Algorithm Tutorials

QuickSort is a sorting algorithm based on the Divide and Conquer algorithm that picks an element as a pivot and partitions the given array around the picked pivot by placing the pivot in its correct position in the sorted array.

How does QuickSort work?

The key process in quickSort is a partition(). The target of partitions is to place the pivot (any element can be chosen to be a pivot) at its correct position in the sorted array and put all smaller elements to the left of the pivot, and all greater elements to the right of the pivot.

Partition is done recursively on each side of the pivot after the pivot is placed in its correct position and this finally sorts the array.

How Quicksort works

Choice of Pivot:

There are many different choices for picking pivots. 

• Always pick the first element as a pivot.
• Always pick the last element as a pivot (implemented below)
• Pick a random element as a pivot.
• Pick the middle as the pivot.

Partition Algorithm:

The logic is simple, we start from the leftmost element and keep track of the index of smaller (or equal) elements as i. While traversing, if we find a smaller element, we swap the current element with arr[i]. Otherwise, we ignore the current element.

Let us understand the working of partition and the Quick Sort algorithm with the help of the following example:

Consider: arr[] = {10, 80, 30, 90, 40}.

• Compare 10 with the pivot and as it is less than pivot arrange it accrodingly.

Partition in QuickSort: Compare pivot with 10

• Compare 80 with the pivot. It is greater than pivot.

Partition in QuickSort: Compare pivot with 80

• Compare 30 with pivot. It is less than pivot so arrange it accordingly.

Partition in QuickSort: Compare pivot with 30

• Compare 90 with the pivot. It is greater than the pivot.

Partition in QuickSort: Compare pivot with 90

• Arrange the pivot in its correct position.

Partition in QuickSort: Place pivot in its correct position

Illustration of Quicksort:

As the partition process is done recursively, it keeps on putting the pivot in its actual position in the sorted array. Repeatedly putting pivots in their actual position makes the array sorted.

Follow the below images to understand how the recursive implementation of the partition algorithm helps to sort the array.

• Initial partition on the main array:

Quicksort: Performing the partition

• Partitioning of the subarrays:

Quicksort: Performing the partition

Code implementation of the Quick Sort:

Below is the implementation of the Quicksort:

C

// C code to implement quicksort

#include <stdio.h>

// Function to swap two elements
void swap(int* a, int* b)
{
    int t = *a;
    *a = *b;
    *b = t;
}

// Partition the array using the last element as the pivot
int partition(int arr[], int low, int high)
{
    // Choosing the pivot
    int pivot = arr[high];

    // Index of smaller element and indicates
    // the right position of pivot found so far
    int i = (low - 1);

    for (int j = low; j <= high - 1; j++) {

        // If current element is smaller than the 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);
}

// 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(arr, low, high);

        // Separately sort elements before
        // partition and after partition
        quickSort(arr, low, pi - 1);
        quickSort(arr, pi + 1, high);
    }
}

// Driver code
int main()
{
    int arr[] = { 10, 7, 8, 9, 1, 5 };
    int N = sizeof(arr) / sizeof(arr[0]);

    // Function call
    quickSort(arr, 0, N - 1);
    printf("Sorted array: \n");
    for (int i = 0; i < N; i++)
        printf("%d ", arr[i]);
    return 0;
}

C++

// C++ code to implement quicksort

#include <bits/stdc++.h>
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 to left
// of pivot and all greater elements to right of pivot
int partition(int arr[], int low, int high)
{
    // Choosing the pivot
    int pivot = arr[high];

    // Index of smaller element and indicates
    // the right position of pivot found so far
    int i = (low - 1);

    for (int j = low; j <= high - 1; j++) {

        // If current element is smaller than the 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);
}

// 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(arr, low, high);

        // Separately sort elements before
        // partition and after partition
        quickSort(arr, low, pi - 1);
        quickSort(arr, pi + 1, high);
    }
}

// Driver Code
int main()
{
    int arr[] = { 10, 7, 8, 9, 1, 5 };
    int N = sizeof(arr) / sizeof(arr[0]);

    // Function call
    quickSort(arr, 0, N - 1);
    cout << "Sorted array: " << endl;
    for (int i = 0; i < N; i++)
        cout << arr[i] << " ";
    return 0;
}

Java

// Java implementation of QuickSort
import java.io.*;

class GFG {

    // A utility function to swap two elements
    static void swap(int[] arr, int i, int j)
    {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }

    // This function takes last element as pivot,
    // places the pivot element at its correct position
    // in sorted array, and places all smaller to left
    // of pivot and all greater elements to right of pivot
    static int partition(int[] arr, int low, int high)
    {
        // Choosing the pivot
        int pivot = arr[high];

        // Index of smaller element and indicates
        // the right position of pivot found so far
        int i = (low - 1);

        for (int j = low; j <= high - 1; j++) {

            // If current element is smaller than the pivot
            if (arr[j] < pivot) {

                // Increment index of smaller element
                i++;
                swap(arr, i, j);
            }
        }
        swap(arr, i + 1, high);
        return (i + 1);
    }

    // The main function that implements QuickSort
    // arr[] --> Array to be sorted,
    // low --> Starting index,
    // high --> Ending index
    static 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(arr, low, high);

            // Separately sort elements before
            // partition and after partition
            quickSort(arr, low, pi - 1);
            quickSort(arr, pi + 1, high);
        }
    }
    // To print sorted array
    public static void printArr(int[] arr)
    {
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }

    // Driver Code
    public static void main(String[] args)
    {
        int[] arr = { 10, 7, 8, 9, 1, 5 };
        int N = arr.length;

        // Function call
        quickSort(arr, 0, N - 1);
        System.out.println("Sorted array:");
        printArr(arr);
    }
}

// This code is contributed by Ayush Choudhary
// Improved by Ajay Virmoti

Python3

# Python3 implementation of QuickSort


# Function to find the partition position
def partition(array, low, high):

    # Choose the rightmost element as pivot
    pivot = array[high]

    # Pointer for greater element
    i = low - 1

    # Traverse through all elements
    # compare each element with pivot
    for j in range(low, high):
        if array[j] <= pivot:

            # If element smaller than pivot is found
            # swap it with the greater element pointed by i
            i = i + 1

            # Swapping element at i with element at j
            (array[i], array[j]) = (array[j], array[i])

    # Swap the pivot element with
    # the greater element specified by i
    (array[i + 1], array[high]) = (array[high], array[i + 1])

    # Return the position from where partition is done
    return i + 1


# Function to perform quicksort
def quicksort(array, low, high):
    if low < high:

        # Find pivot element such that
        # element smaller than pivot are on the left
        # element greater than pivot are on the right
        pi = partition(array, low, high)

        # Recursive call on the left of pivot
        quicksort(array, low, pi - 1)

        # Recursive call on the right of pivot
        quicksort(array, pi + 1, high)


# Driver code
if __name__ == '__main__':
    array = [10, 7, 8, 9, 1, 5]
    N = len(array)

    # Function call
    quicksort(array, 0, N - 1)
    print('Sorted array:')
    for x in array:
        print(x, end=" ")

# This code is contributed by Adnan Aliakbar

C#

// C# implementation of QuickSort

using System;

class GFG {

    // A utility function to swap two elements
    static void swap(int[] arr, int i, int j)
    {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }

    // This function takes last element as pivot,
    // places the pivot element at its correct position
    // in sorted array, and places all smaller to left
    // of pivot and all greater elements to right of pivot
    static int partition(int[] arr, int low, int high)
    {
        // Choosing the pivot
        int pivot = arr[high];

        // Index of smaller element and indicates
        // the right position of pivot found so far
        int i = (low - 1);

        for (int j = low; j <= high - 1; j++) {

            // If current element is smaller than the pivot
            if (arr[j] < pivot) {

                // Increment index of smaller element
                i++;
                swap(arr, i, j);
            }
        }
        swap(arr, i + 1, high);
        return (i + 1);
    }

    // The main function that implements QuickSort
    // arr[] --> Array to be sorted,
    // low --> Starting index,
    // high --> Ending index
    static 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(arr, low, high);

            // Separately sort elements before
            // and after partition index
            quickSort(arr, low, pi - 1);
            quickSort(arr, pi + 1, high);
        }
    }

    // Driver Code
    public static void Main()
    {
        int[] arr = { 10, 7, 8, 9, 1, 5 };
        int N = arr.Length;

        // Function call
        quickSort(arr, 0, N - 1);
        Console.WriteLine("Sorted array:");
        for (int i = 0; i < N; i++)
            Console.Write(arr[i] + " ");
    }
}

// This code is contributed by gfgking

Output

Sorted array: 
1 5 7 8 9 10 

Complexity Analysis of Quick Sort:

Time Complexity:

• Best Case: 
• Average Case:
• Worst Case: O(N²)

Auxiliary Space: O(1) as no extra space is used

Advantages of Quick Sort:

• It is a divide-and-conquer algorithm that makes it easier to solve problems.
• It is efficient on large data sets.
• It has a low overhead, as it only requires a small amount of memory to function.

Disadvantages of Quick Sort:

• It has a worst-case time complexity of O(N²), which occurs when the pivot is chosen poorly.
• It is not a good choice for small data sets.
• It is not a stable sort, meaning that if two elements have the same key, their relative order will not be preserved in the sorted output in case of quick sort, because here we are swapping elements according to the pivot’s position (without considering their original positions).