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How to implement QuickSort in Java

Educative Answers Team

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
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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("]");
    }
}

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quicksort
sorting
java
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