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

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.");
+    }
+}