Add recursive merge sort (#2446)

Sorts/MergeSortRecursive.java

@@ -0,0 +1,72 @@
+import java.util.Arrays;
+import java.util.ArrayList;
+import java.util.List;
+
+public class MergeSortRecursive {
+
+    List<Integer> arr;
+
+    public MergeSortRecursive(List<Integer> arr) {
+        this.arr = arr;
+    }
+
+    public void mergeSort() {
+        List<Integer> arrSorted = merge(arr);
+        System.out.println(arrSorted);
+    }
+
+    private static List<Integer> merge(List<Integer> arr) {
+
+        // base condition
+        if (arr.size() <= 1) {
+            return arr;
+        }
+
+        int arrLength = arr.size();
+        int half = arrLength / 2;
+        List<Integer> arrA = arr.subList(0, half);
+        List<Integer> arrB = arr.subList(half, arr.size());
+
+        // recursion
+        arrA = merge(arrA);
+        arrB = merge(arrB);
+
+        return sort(arrA, arrB);
+    }
+
+    private static List<Integer> sort(List<Integer> unsortedA, List<Integer> unsortedB) {
+        if (unsortedA.size() <= 0 && unsortedB.size() <= 0) {
+            return new ArrayList<>();
+        }
+        if (unsortedA.size() <= 0) {
+            return unsortedB;
+        }
+        if (unsortedB.size() <= 0) {
+            return unsortedA;
+        }
+        if (unsortedA.get(0) <= unsortedB.get(0)) {
+            List<Integer> newAl = new ArrayList<Integer>() {
+                {
+                    add(unsortedA.get(0));
+                }
+            };
+            newAl.addAll(sort(unsortedA.subList(1, unsortedA.size()), unsortedB));
+            return newAl;
+        } else {
+            List<Integer> newAl = new ArrayList<Integer>() {
+                {
+                    add(unsortedB.get(0));
+                }
+            };newAl.addAll(sort(unsortedA, unsortedB.subList(1, unsortedB.size())));
+            return newAl;
+        }
+}
+
+}
+
+class App {
+    public static void main(String[] args) {
+        MergeSortRecursive sort = new MergeSortRecursive(new ArrayList<>(Arrays.asList(4, 3, 1, 8, 5, 10, 0, 1, 4, 11, 8, 9)));
+        sort.mergeSort();
+    }
+}