Trusted answers to developer questions
Trusted Answers to Developer Questions

Related Tags

quicksort
sorting
java

How to implement QuickSort in Java

Educative Answers Team

Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

QuickSort is an in-place sorting algorithm with worst-case time complexity of n2n^{2}


Implementation

​​QuickSort can be implemented iteratively and recursively. We’ll mainly focus on the recursive implementation, as it is far more convenient, intuitive and simplistic. Iterative implementation is generally unrecommended. Arrays are used as an example here, but it can be implemented on other data structures (like linked lists) as well.

The algorithm can be broken down into 3 parts.

  1. Partitioning the array about the pivot
  2. Passing the smaller arrays to the recursive calls
  3. Joining the sorted arrays that are returned from the recursive call, and the pivot
1 of 16

In the above illustration, we used the first element of the ​array as a pivot (though any of the elements can be taken).

Code

import java.util.*;
class Quick_Sort {
public static int[] QuickSort(int[] arr, int elements) {
if(elements < 2){ //Base Case
return arr;
}
int current_position=0; //position of pivot element
int temp; //a temporary variable to assist in swapping
for(int i=1; i<elements; i++) //Partitioning loop
{
if(arr[i] <= arr[0])
{
current_position++;
temp = arr[i];
arr[i] = arr[current_position];
arr[current_position] = temp;
}
}
temp = arr[0];
arr[0] = arr[current_position];
arr[current_position] = temp; //Brings pivot to it's appropriate position
int[] left = QuickSort(arr,current_position); //sorts the elements to the left of pivot
int[] arr2 = Arrays.copyOfRange(arr, current_position+1, elements);//separates elements right of pivot
int[] right = QuickSort(arr2, elements-current_position-1); //sorts the elements to the right of pivot
int[] final_array = new int[elements]; //final array, to merge everything together
for(int i=0; i<current_position; i++)
{
final_array[i] = left[i];
}
final_array[current_position] = arr[current_position];
for(int i=current_position+1; i<elements; i++)
{
final_array[i] = right[i-current_position-1];
}
return final_array;
}
public static void main( String args[] ) {
int[] array = {4,2,7,3,1,6}; //array to be sorted
System.out.print("Original Array: [");
for(int i=0; i<array.length;i++)
{
System.out.print(array[i]);
System.out.print(" ");
}
System.out.println("]");
array = QuickSort(array, array.length);//sorting
System.out.print("Sorted Array: ["); //printing the sorted array
for(int i=0; i<array.length;i++)
{
System.out.print(array[i]);
System.out.print(" ");
}
System.out.print("]");
}
}

RELATED TAGS

quicksort
sorting
java
Copyright ©2022 Educative, Inc. All rights reserved

Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

Keep Exploring