Java Program to Implement Quick Sort Algorithm

Quicksort in Java

Quicksort algorithm is based on the divide and conquer approach where an array is divided into subarrays by selecting a pivot element.

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.

The same process is continued for both left and right subarrays. Finally, sorted elements are combined to form a sorted array.

To learn more, visit Quicksort Algorithm.

Example: Java Program to Implement Quick Sort Algorithm

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 greater 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));
  }
}

Output

Unsorted Array
[8, 7, 2, 1, 0, 9, 6]
Sorted Array in Ascending Order
[0, 1, 2, 6, 7, 8, 9]

Here, the elements of the array are sorted in ascending order. If we want to sort the elements in descending order, then inside the for loop, we can change the code as:

// the less than sign is changed to greater than
if (array[j] >= pivot) {