Add CountingInversions algorithm (#5745)

src/test/java/com/thealgorithms/divideandconquer/CountingInversionsTest.java

@@ -0,0 +1,32 @@
+package com.thealgorithms.divideandconquer;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class CountingInversionsTest {
+
+    @Test
+    public void testCountInversions() {
+        int[] arr = {2, 3, 8, 6, 1};
+        assertEquals(5, CountingInversions.countInversions(arr));
+    }
+
+    @Test
+    public void testNoInversions() {
+        int[] arr = {1, 2, 3, 4, 5};
+        assertEquals(0, CountingInversions.countInversions(arr));
+    }
+
+    @Test
+    public void testSingleElement() {
+        int[] arr = {1};
+        assertEquals(0, CountingInversions.countInversions(arr));
+    }
+
+    @Test
+    public void testAllInversions() {
+        int[] arr = {5, 4, 3, 2, 1};
+        assertEquals(10, CountingInversions.countInversions(arr));
+    }
+}