Quicksort is
a divide and conquer algorithm. Quicksort first divides a large array into two
smaller sub-arrays: the low elements and the high elements. Quicksort can then
recursively sort the sub-arrays.
The steps
are:
a.
Pick
an element, called a pivot, from the array.
b.
Reorder
the array so that all elements with values less than the pivot come before the
pivot, while all elements with values greater than the pivot come after it
(equal values can go either way). After this partitioning, the pivot is in its
final position. This is called the partition operation.
c.
Recursively
apply the above steps to the sub-array of elements with smaller values and
separately to the sub-array of elements with greater values.
Following
video explains about quick sort clearly.
import java.util.Arrays; import java.util.Objects; public class QuickSort<T extends Comparable<T>> { private T[] arr; private boolean flag; private void swap(int i, int j) { T temp = arr[i]; arr[i] = arr[j]; arr[j] = temp; } private void quickSort(int low, int high) { if (low < high) { int position = partition(low, high); quickSort(low, position); quickSort(position + 1, high); } } private boolean compare(T i, T j) { if (flag == true) return i.compareTo(j) < 0; return i.compareTo(j) > 0; } private int partition(int low, int high) { int i = low + 1; T pivot = arr[low]; for (int j = low + 1; j < high; j++) { if (compare(arr[j], pivot)) { swap(i, j); i++; } } i--; swap(low, i); return i; } /** * If flag is true, then elements are sorted in ascending order, else * descending order */ public void sort(T arr[], boolean flag) { Objects.nonNull(arr); this.arr = arr; this.flag = flag; quickSort(0, arr.length); } public static void main(String args[]) { Integer arr[] = { 1, 3, 5, 7, 2, 4, 6, 8 }; QuickSort<Integer> quickSort = new QuickSort<>(); quickSort.sort(arr, true); System.out.println(Arrays.toString(arr)); quickSort.sort(arr, false); System.out.println(Arrays.toString(arr)); } }
Following is
the junit test case for above program.
import static org.junit.Assert.assertTrue; import java.util.Arrays; import org.junit.Test; public class QuickSortTest { @Test public void test1() { QuickSort<Integer> obj1 = new QuickSort<>(); Integer arr1[] = { 5, 46, 25, 13, 12 }; Integer arr2[] = { 5, 46 }; Integer arr3[] = { 5, 4, 3, 2, 1 }; Integer arr4[] = { 1 }; Integer arr5[] = { 1, 3, 5, 7, 2, 4, 6, 8 }; obj1.sort(arr1, false); obj1.sort(arr2, false); obj1.sort(arr3, false); obj1.sort(arr4, false); obj1.sort(arr5, false); assertTrue(Arrays.equals(arr1, new Integer[] { 46, 25, 13, 12, 5 })); assertTrue(Arrays.equals(arr2, new Integer[] { 46, 5 })); assertTrue(Arrays.equals(arr3, new Integer[] { 5, 4, 3, 2, 1 })); assertTrue(Arrays.equals(arr4, new Integer[] { 1 })); assertTrue(Arrays .equals(arr5, new Integer[] { 8, 7, 6, 5, 4, 3, 2, 1 })); } @Test public void test2() { QuickSort<Integer> obj1 = new QuickSort<>(); Integer arr1[] = { 5, 46, 25, 13, 12 }; Integer arr2[] = { 5, 46 }; Integer arr3[] = { 5, 4, 3, 2, 1 }; Integer arr4[] = { 1 }; Integer arr5[] = { 1, 3, 5, 7, 2, 4, 6, 8 }; obj1.sort(arr1, true); obj1.sort(arr2, true); obj1.sort(arr3, true); obj1.sort(arr4, true); obj1.sort(arr5, true); assertTrue(Arrays.equals(arr1, new Integer[] { 5, 12, 13, 25, 46 })); assertTrue(Arrays.equals(arr2, new Integer[] { 5, 46 })); assertTrue(Arrays.equals(arr3, new Integer[] { 1, 2, 3, 4, 5 })); assertTrue(Arrays.equals(arr4, new Integer[] { 1 })); assertTrue(Arrays .equals(arr5, new Integer[] { 1, 2, 3, 4, 5, 6, 7, 8 })); } }
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