quicksort.java Add Quicksort Algorithm Closes NITSkmOS#150

README.md

@@ -16,7 +16,7 @@ This repository contains examples of various algorithms written on different pro
 |:----------------------------------------------------------------------------------------------- |:-------------------------------------:|:-------------------------------------:|:-------------------------------------:|:-------------------------------------:|
 | [Euclidean GCD](https://en.wikipedia.org/wiki/Euclidean_algorithm)                              | [:octocat:](euclidean_gcd/C)          |                                       | [:octocat:](euclidean_gcd/Java)       | [:octocat:](euclidean_gcd/Python)     |
 | [Merge Sort](https://en.wikipedia.org/wiki/Merge_sort)                                          | [:octocat:](merge_sort/C)             | [:octocat:](merge_sort/Cpp)           | [:octocat:](merge_sort/Java)          | [:octocat:](merge_sort/Python)        |
-| [QuickSort](https://en.wikipedia.org/wiki/Quicksort)                                            | [:octocat:](quicksort/C)              |                                       |                                       | [:octocat:](quicksort/Python)         |
+| [QuickSort](https://en.wikipedia.org/wiki/Quicksort)                                            | [:octocat:](quicksort/C)              |                                       | [:octocat:](quicksort/Java)           | [:octocat:](quicksort/Python)         |
 | [Insertion Sort](https://en.wikipedia.org/wiki/Insertion_sort)                                  | [:octocat:](insertion_sort/C)         | [:octocat:](insertion_sort/Cpp)       |                                       | [:octocat:](insertion_sort/Python)    |
 | [Counting Sort](https://en.wikipedia.org/wiki/Counting_sort)                                    | [:octocat:](counting_sort/C)          | [:octocat:](counting_sort/Cpp)        |                                       | [:octocat:](counting_sort/Python)     |
 | [Radix Sort](https://en.wikipedia.org/wiki/Radix_sort)                                          |                                       | [:octocat:](radix_sort/Cpp)           |                                       | [:octocat:](radix_sort/Python)        |

quicksort/Java/quicksort.java

@@ -0,0 +1,51 @@
+import java.util.Arrays;
+
+class quicksort {
+    //Main function. Instanciates the unsorted array and calls quicksort.
+    public static void main(String args[]) {
+        int array[] = {9, 5, 4, 6, 1, 2, 3, 1, 2, 3};
+        System.out.println("The unsorted array is:");
+        System.out.println(Arrays.toString(array));
+
+        quicksort(array, 0, array.length - 1);
+
+        System.out.println("The sorted array is:");
+        System.out.println(Arrays.toString(array));
+    }
+
+    //partition returns the array splitted on
+    // lesser and greater values than a pivot chosen randomly.
+    //(on this case, the array's first value)
+    //Then, quicksort is called recursively on both splits.
+    static void quicksort(int array[], int start, int end) {
+        if (start < end) {
+            int pIndex = partition(array, start, end);
+            quicksort(array, start, pIndex - 1);
+            quicksort(array, pIndex + 1, end);
+        }
+    }
+
+    //Defines the pivot index at the start of array.
+    //Finds every element with lesser value and swaps places.
+    //At the end of the cycle:
+    // pivot splits the array on lesser and greater values.
+    static int partition(int array[], int start, int end) {
+        int pivot = array[end];
+        int pIndex = start;
+        for (int i = start; i < end; i++) {
+            if (array[i] <= pivot) {
+                swap(array, i, pIndex);
+                pIndex++;
+            }
+        }
+        swap(array, pIndex, end);
+        return pIndex;
+    }
+
+    //Swaps indexes 'a' and 'b' on array
+    static void swap(int array[], int a, int b) {
+        int temp = array[a];
+        array[a] = array[b];
+        array[b] = temp;
+    }
+}