testing: improving CountSinglyLinkedListRecursionTest (#6413)

Co-authored-by: Deniz Altunkapan <93663085+DenizAltunkapan@users.noreply.github.com>

src/test/java/com/thealgorithms/datastructures/lists/CountSinglyLinkedListRecursionTest.java

@@ -1,8 +1,10 @@
 package com.thealgorithms.datastructures.lists;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
 import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
 public class CountSinglyLinkedListRecursionTest {
@@ -15,70 +17,112 @@ public class CountSinglyLinkedListRecursionTest {
     }
 
     @Test
+    @DisplayName("Count of an empty list should be 0")
     public void testCountEmptyList() {
-        // An empty list should have a count of 0
-        assertEquals(0, list.count(), "Count of an empty list should be 0.");
+        assertEquals(0, list.count());
     }
 
     @Test
+    @DisplayName("Count after inserting a single element should be 1")
     public void testCountSingleElementList() {
-        // Insert a single element and check the count
         list.insert(1);
-        assertEquals(1, list.count(), "Count of a single-element list should be 1.");
+        assertEquals(1, list.count());
     }
 
     @Test
+    @DisplayName("Count after inserting multiple distinct elements")
     public void testCountMultipleElements() {
-        // Insert multiple elements and check the count
         for (int i = 1; i <= 5; i++) {
             list.insert(i);
         }
-        assertEquals(5, list.count(), "Count of a list with 5 elements should be 5.");
+        assertEquals(5, list.count());
     }
 
     @Test
+    @DisplayName("Count should reflect total number of nodes with duplicate values")
     public void testCountWithDuplicateElements() {
-        // Insert duplicate elements and verify the count is correct
+        list.insert(2);
+        list.insert(2);
+        list.insert(3);
+        list.insert(3);
         list.insert(1);
-        list.insert(2);
-        list.insert(2);
-        list.insert(3);
-        list.insert(3);
-        assertEquals(5, list.count(), "Count of a list with duplicate elements should match total node count.");
+        assertEquals(5, list.count());
     }
 
     @Test
+    @DisplayName("Count should return 0 after clearing the list")
     public void testCountAfterClearingList() {
         for (int i = 1; i <= 4; i++) {
             list.insert(i);
         }
-        list.clear(); // assuming you have a clear method; if not, skip this
-        assertEquals(0, list.count(), "Count after clearing the list should be 0.");
+        list.clear(); // assumed to exist
+        assertEquals(0, list.count());
     }
 
     @Test
+    @DisplayName("Count on a very large list should be accurate")
     public void testCountOnVeryLargeList() {
         int n = 1000;
         for (int i = 0; i < n; i++) {
             list.insert(i);
         }
-        assertEquals(n, list.count(), "Count should correctly return for large list sizes.");
+        assertEquals(n, list.count());
     }
 
     @Test
+    @DisplayName("Count should work correctly with negative values")
     public void testCountOnListWithNegativeNumbers() {
         list.insert(-1);
-        list.insert(-5);
-        list.insert(-10);
-        assertEquals(3, list.count(), "Count should correctly handle negative values.");
+        list.insert(-2);
+        list.insert(-3);
+        assertEquals(3, list.count());
     }
 
     @Test
+    @DisplayName("Calling count multiple times should return the same value if list is unchanged")
     public void testCountIsConsistentWithoutModification() {
         list.insert(1);
         list.insert(2);
-        int firstCount = list.count();
-        int secondCount = list.count();
-        assertEquals(firstCount, secondCount, "Repeated count calls should return consistent values.");
+        int count1 = list.count();
+        int count2 = list.count();
+        assertEquals(count1, count2);
+    }
+
+    @Test
+    @DisplayName("Count should reflect total even if all values are the same")
+    public void testCountAllSameValues() {
+        for (int i = 0; i < 5; i++) {
+            list.insert(42);
+        }
+        assertEquals(5, list.count());
+    }
+
+    @Test
+    @DisplayName("Count should remain correct after multiple interleaved insert and count operations")
+    public void testCountAfterEachInsert() {
+        assertEquals(0, list.count());
+        list.insert(1);
+        assertEquals(1, list.count());
+        list.insert(2);
+        assertEquals(2, list.count());
+        list.insert(3);
+        assertEquals(3, list.count());
+    }
+
+    @Test
+    @DisplayName("List should not throw on edge count (0 nodes)")
+    public void testEdgeCaseNoElements() {
+        assertDoesNotThrow(() -> list.count());
+    }
+
+    @Test
+    @DisplayName("Should count accurately after inserting then removing all elements")
+    public void testCountAfterInsertAndClear() {
+        for (int i = 0; i < 10; i++) {
+            list.insert(i);
+        }
+        assertEquals(10, list.count());
+        list.clear();
+        assertEquals(0, list.count());
     }
 }