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Quicksort Algorithm

In this tutorial, you will learn about the quick sort algorithm and its implementation in Python, Java, C, and C++.

Quicksort is a sorting algorithm based on the divide and conquer approach where

  1. An array is divided into subarrays by selecting a pivot element (element selected from the array).

    While dividing the array, the pivot element should be positioned in such a way that elements less than pivot are kept on the left side and elements greater than pivot are on the right side of the pivot.
  2. The left and right subarrays are also divided using the same approach. This process continues until each subarray contains a single element.
  3. At this point, elements are already sorted. Finally, elements are combined to form a sorted array.

Working of Quicksort Algorithm

1. Select the Pivot Element

There are different variations of quicksort where the pivot element is selected from different positions. Here, we will be selecting the rightmost element of the array as the pivot element.

Quick Sort Steps
Select a pivot element

2. Rearrange the Array

Now the elements of the array are rearranged so that elements that are smaller than the pivot are put on the left and the elements greater than the pivot are put on the right.

Quick Sort Steps
Put all the smaller elements on the left and greater on the right of pivot element

Here's how we rearrange the array:

  1. A pointer is fixed at the pivot element. The pivot element is compared with the elements beginning from the first index.
    Quick Sort Steps
    Comparison of pivot element with element beginning from the first index
  2. If the element is greater than the pivot element, a second pointer is set for that element.
    Quick Sort Steps
    If the element is greater than the pivot element, a second pointer is set for that element.
  3. Now, pivot is compared with other elements. If an element smaller than the pivot element is reached, the smaller element is swapped with the greater element found earlier.
    Quick Sort Steps
    Pivot is compared with other elements.
  4. Again, the process is repeated to set the next greater element as the second pointer. And, swap it with another smaller element.
    Quick Sort Steps
    The process is repeated to set the next greater element as the second pointer.
  5. The process goes on until the second last element is reached.
    Quick Sort Steps
    The process goes on until the second last element is reached.
  6. Finally, the pivot element is swapped with the second pointer.
    Quick Sort Steps
    Finally, the pivot element is swapped with the second pointer.

3. Divide Subarrays

Pivot elements are again chosen for the left and the right sub-parts separately. And, step 2 is repeated.

Quick Sort Steps
Select pivot element of in each half and put at correct place using recursion

 

The subarrays are divided until each subarray is formed of a single element. At this point, the array is already sorted.


Quick Sort Algorithm


quickSort(array, leftmostIndex, rightmostIndex)
  if (leftmostIndex < rightmostIndex)
    pivotIndex <- partition(array,leftmostIndex, rightmostIndex)
    quickSort(array, leftmostIndex, pivotIndex - 1)
    quickSort(array, pivotIndex, rightmostIndex)

partition(array, leftmostIndex, rightmostIndex)
  set rightmostIndex as pivotIndex
  storeIndex <- leftmostIndex - 1
  for i <- leftmostIndex + 1 to rightmostIndex
  if element[i] < pivotElement
    swap element[i] and element[storeIndex]
    storeIndex++
  swap pivotElement and element[storeIndex+1]
return storeIndex + 1

Visual Illustration of Quicksort Algorithm

You can understand the working of quicksort algorithm with the help of the illustrations below.

Quick Sort Steps
Sorting the elements on the left of pivot using recursion
Quick Sort Steps
Sorting the elements on the right of pivot using recursion


Quicksort Code in Python, Java, and C/C++

# Quick sort in Python

# 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)


data = [8, 7, 2, 1, 0, 9, 6]
print("Unsorted Array")
print(data)

size = len(data)

quickSort(data, 0, size - 1)

print('Sorted Array in Ascending Order:')
print(data)
// Quick sort in Java

import java.util.Arrays;

class Quicksort {

  // method to find the partition position
  static int partition(int array[], int low, int high) {
    
    // choose the rightmost element as pivot
    int pivot = array[high];
    
    // pointer for greater element
    int i = (low - 1);

    // traverse through all elements
    // compare each element with pivot
    for (int j = low; j < high; j++) {
      if (array[j] <= pivot) {

        // if element smaller than pivot is found
        // swap it with the greatr element pointed by i
        i++;

        // swapping element at i with element at j
        int temp = array[i];
        array[i] = array[j];
        array[j] = temp;
      }

    }

    // swapt the pivot element with the greater element specified by i
    int temp = array[i + 1];
    array[i + 1] = array[high];
    array[high] = temp;

    // return the position from where partition is done
    return (i + 1);
  }

  static void quickSort(int array[], int low, int high) {
    if (low < high) {

      // find pivot element such that
      // elements smaller than pivot are on the left
      // elements greater than pivot are on the right
      int 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);
    }
  }
}

// Main class
class Main {
  public static void main(String args[]) {

    int[] data = { 8, 7, 2, 1, 0, 9, 6 };
    System.out.println("Unsorted Array");
    System.out.println(Arrays.toString(data));

    int size = data.length;

    // call quicksort() on array data
    Quicksort.quickSort(data, 0, size - 1);

    System.out.println("Sorted Array in Ascending Order: ");
    System.out.println(Arrays.toString(data));
  }
}
// Quick sort in C

#include <stdio.h>

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

// function to find the partition position
int partition(int array[], int low, int high) {
  
  // select the rightmost element as pivot
  int pivot = array[high];
  
  // pointer for greater element
  int i = (low - 1);

  // traverse each element of the array
  // compare them with the pivot
  for (int j = low; j < high; j++) {
    if (array[j] <= pivot) {
        
      // if element smaller than pivot is found
      // swap it with the greater element pointed by i
      i++;
      
      // swap element at i with element at j
      swap(&array[i], &array[j]);
    }
  }

  // swap the pivot element with the greater element at i
  swap(&array[i + 1], &array[high]);
  
  // return the partition point
  return (i + 1);
}

void quickSort(int array[], int low, int high) {
  if (low < high) {
    
    // find the pivot element such that
    // elements smaller than pivot are on left of pivot
    // elements greater than pivot are on right of pivot
    int 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);
  }
}

// function to print array elements
void printArray(int array[], int size) {
  for (int i = 0; i < size; ++i) {
    printf("%d  ", array[i]);
  }
  printf("\n");
}

// main function
int main() {
  int data[] = {8, 7, 2, 1, 0, 9, 6};
  
  int n = sizeof(data) / sizeof(data[0]);
  
  printf("Unsorted Array\n");
  printArray(data, n);
  
  // perform quicksort on data
  quickSort(data, 0, n - 1);
  
  printf("Sorted array in ascending order: \n");
  printArray(data, n);
}
// Quick sort in C++

#include <iostream>
using namespace std;

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

// function to print the array
void printArray(int array[], int size) {
  int i;
  for (i = 0; i < size; i++)
    cout << array[i] << " ";
  cout << endl;
}

// function to rearrange array (find the partition point)
int partition(int array[], int low, int high) {
    
  // select the rightmost element as pivot
  int pivot = array[high];
  
  // pointer for greater element
  int i = (low - 1);

  // traverse each element of the array
  // compare them with the pivot
  for (int j = low; j < high; j++) {
    if (array[j] <= pivot) {
        
      // if element smaller than pivot is found
      // swap it with the greater element pointed by i
      i++;
      
      // swap element at i with element at j
      swap(&array[i], &array[j]);
    }
  }
  
  // swap pivot with the greater element at i
  swap(&array[i + 1], &array[high]);
  
  // return the partition point
  return (i + 1);
}

void quickSort(int array[], int low, int high) {
  if (low < high) {
      
    // find the pivot element such that
    // elements smaller than pivot are on left of pivot
    // elements greater than pivot are on righ of pivot
    int 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
int main() {
  int data[] = {8, 7, 6, 1, 0, 9, 2};
  int n = sizeof(data) / sizeof(data[0]);
  
  cout << "Unsorted Array: \n";
  printArray(data, n);
  
  // perform quicksort on data
  quickSort(data, 0, n - 1);
  
  cout << "Sorted array in ascending order: \n";
  printArray(data, n);
}

Quicksort Complexity

Time Complexity  
Best O(n*log n)
Worst O(n2)
Average O(n*log n)
Space Complexity O(log n)
Stability No

1. Time Complexities

  • Worst Case Complexity [Big-O]: O(n2)
    It occurs when the pivot element picked is either the greatest or the smallest element.

    This condition leads to the case in which the pivot element lies in an extreme end of the sorted array. One sub-array is always empty and another sub-array contains n - 1 elements. Thus, quicksort is called only on this sub-array.

    However, the quicksort algorithm has better performance for scattered pivots.
  • Best Case Complexity [Big-omega]: O(n*log n)
    It occurs when the pivot element is always the middle element or near to the middle element.
  • Average Case Complexity [Big-theta]: O(n*log n)
    It occurs when the above conditions do not occur.

2. Space Complexity

The space complexity for quicksort is O(log n).


Quicksort Applications

Quicksort algorithm is used when

  • the programming language is good for recursion
  • time complexity matters
  • space complexity matters

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