Quicksort Array in Java
Quicksort is a divide and conquer algorithm. It first divides a large list into two smaller sublists and then recursively sort the two sublists. If we want to sort an array without any extra space, quicksort is a good option. On average, time complexity is O(n log(n)).
The basic step of sorting an array are as follows:
 Select a pivot, normally the middle one
 From both ends, swap elements and make left elements < pivot and all right > pivot
 Recursively sort left part and right part
Here is a very good animation of quicksort.
public class QuickSort { public static void main(String[] args) { int[] x = { 9, 2, 4, 7, 3, 7, 10 }; System.out.println(Arrays.toString(x)); int low = 0; int high = x.length  1; quickSort(x, low, high); System.out.println(Arrays.toString(x)); } public static void quickSort(int[] arr, int low, int high) { if (arr == null  arr.length == 0) return; if (low >= high) return; // pick the pivot int middle = low + (high  low) / 2; int pivot = arr[middle]; // make left < pivot and right > pivot int i = low, j = high; while (i <= j) { while (arr[i] < pivot) { i++; } while (arr[j] > pivot) { j; } if (i <= j) { int temp = arr[i]; arr[i] = arr[j]; arr[j] = temp; i++; j; } } // recursively sort two sub parts if (low < j) quickSort(arr, low, j); if (high > i) quickSort(arr, i, high); } } 
Output:
9 2 4 7 3 7 10
2 3 4 7 7 9 10
2 3 4 7 7 9 10
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