testing: improving test coverage CountingInversionsTest (#6393)

testing: improving test coverage CountingInversionsTest

Co-authored-by: Deniz Altunkapan <[email protected]>

Author: alxkm

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

@@ -29,4 +29,35 @@ public void testAllInversions() {
         int[] arr = {5, 4, 3, 2, 1};
         assertEquals(10, CountingInversions.countInversions(arr));
     }
+
+    @Test
+    public void testEmptyArray() {
+        int[] arr = {};
+        assertEquals(0, CountingInversions.countInversions(arr));
+    }
+
+    @Test
+    public void testArrayWithDuplicates() {
+        int[] arr = {1, 3, 2, 3, 1};
+        // Inversions: (3,2), (3,1), (3,1), (2,1)
+        assertEquals(4, CountingInversions.countInversions(arr));
+    }
+
+    @Test
+    public void testLargeArray() {
+        int n = 1000;
+        int[] arr = new int[n];
+        for (int i = 0; i < n; i++) {
+            arr[i] = n - i; // descending order -> max inversions = n*(n-1)/2
+        }
+        int expected = n * (n - 1) / 2;
+        assertEquals(expected, CountingInversions.countInversions(arr));
+    }
+
+    @Test
+    public void testArrayWithAllSameElements() {
+        int[] arr = {7, 7, 7, 7};
+        // No inversions since all elements are equal
+        assertEquals(0, CountingInversions.countInversions(arr));
+    }
 }