Quicksort or partition-exchange sort, is a sorting algorithm, which is using divide and conquer algorithm.

In quick sort, we first choose a pivot and divide into two sublists,one will contain elements lower than pivot and other will have elements greater than pivot.

Best Case : O(n log n)

Average Case : O(n log n)

Worst Case : O(n^2)

In quick sort, we first choose a pivot and divide into two sublists,one will contain elements lower than pivot and other will have elements greater than pivot.

### Quick sort algorithm:

Lets say array is arr[]- Choose a pivot, it is generally mid element of the list.
- Initialise two index variable , left=0 and right=arr.length-1
- Increment left variable until you get element higher than pivot.
- Decrement right variable until you get element lesser than pivot
- swap arr[left] and arr[right]
- Recursively sort sublists(sublist with less than pivot, sublist greater than pivot) using above algorithm.
- In the end , you will get sorted array.

### Java program to implement quick sort:

package org.arpit.java2blog; import java.util.Arrays; public class QuickSortMain { private static int array[]; public static void sort(int[] arr) { if (arr == null || arr.length == 0) { return; } array = arr; quickSort(0, array.length-1); } private static void quickSort(int left, int right) { int i = left; int j = right; // find pivot number, we will take it as mid int pivot = array[left+(right-left)/2]; while (i <= j) { /** * In each iteration, we will increment left until we find element greater than pivot * We will decrement right until we find element less than pivot */ while (array[i] < pivot) { i++; } while (array[j] > pivot) { j--; } if (i <= j) { exchange(i, j); //move index to next position on both sides i++; j--; } } // call quickSort() method recursively if (left < j) quickSort(left, j); if (i < right) quickSort(i, right); } private static void exchange(int i, int j) { int temp = array[i]; array[i] = array[j]; array[j] = temp; } public static void main(String a[]){ int[] input = {33,21,45,64,55,34,11,8,3,5,1}; System.out.println("Before Sorting : "); System.out.println(Arrays.toString(input)); sort(input); System.out.println("=================="); System.out.println("After Sorting : "); System.out.println(Arrays.toString(array)); } }

Before Sorting : [33, 21, 45, 64, 55, 34, 11, 8, 3, 5, 1] ================== After Sorting : [1, 3, 5, 8, 11, 21, 33, 34, 45, 55, 64]

**Time complexity :**Best Case : O(n log n)

Average Case : O(n log n)

Worst Case : O(n^2)