Enhance docs, remove main, add tests in `CountSinglyLinke… (#5992)

DIRECTORY.md

@@ -829,6 +829,7 @@
               * [LeftistHeapTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/heaps/LeftistHeapTest.java)
             * lists
               * [CircleLinkedListTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/CircleLinkedListTest.java)
+              * [CountSinglyLinkedListRecursionTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/CountSinglyLinkedListRecursionTest.java)
               * [CreateAndDetectLoopTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/CreateAndDetectLoopTest.java)
               * [CursorLinkedListTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/CursorLinkedListTest.java)
               * [MergeKSortedLinkedListTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/MergeKSortedLinkedListTest.java)

src/main/java/com/thealgorithms/datastructures/lists/CountSinglyLinkedListRecursion.java

@@ -1,27 +1,26 @@
 package com.thealgorithms.datastructures.lists;
 
+/**
+ * CountSinglyLinkedListRecursion extends a singly linked list to include a
+ * recursive count method, which calculates the number of nodes in the list.
+ */
 public class CountSinglyLinkedListRecursion extends SinglyLinkedList {
 
-    public static void main(String[] args) {
-        CountSinglyLinkedListRecursion list = new CountSinglyLinkedListRecursion();
-        for (int i = 1; i <= 5; ++i) {
-            list.insert(i);
-        }
-        assert list.count() == 5;
-    }
-
     /**
-     * Calculate the count of the list manually using recursion.
+     * Recursively calculates the number of nodes in the list.
      *
-     * @param head head of the list.
+     * @param head the head node of the list segment being counted.
-     * @return count of the list.
+     * @return the count of nodes from the given head node onward.
      */
     private int countRecursion(Node head) {
         return head == null ? 0 : 1 + countRecursion(head.next);
     }
 
     /**
-     * Returns the count of the list.
+     * Returns the total number of nodes in the list by invoking the recursive
+     * count helper method.
+     *
+     * @return the total node count in the list.
      */
     @Override
     public int count() {

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

@@ -0,0 +1,49 @@
+package com.thealgorithms.datastructures.lists;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class CountSinglyLinkedListRecursionTest {
+
+    private CountSinglyLinkedListRecursion list;
+
+    @BeforeEach
+    public void setUp() {
+        list = new CountSinglyLinkedListRecursion();
+    }
+
+    @Test
+    public void testCountEmptyList() {
+        // An empty list should have a count of 0
+        assertEquals(0, list.count(), "Count of an empty list should be 0.");
+    }
+
+    @Test
+    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.");
+    }
+
+    @Test
+    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.");
+    }
+
+    @Test
+    public void testCountWithDuplicateElements() {
+        // Insert duplicate elements and verify the count is correct
+        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.");
+    }
+}