Enhance docs, remove main, add tests in `MergeSortedSingl… (#5997)

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

@@ -0,0 +1,90 @@
+package com.thealgorithms.datastructures.lists;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+class MergeSortedSinglyLinkedListTest {
+
+    @Test
+    void testMergeTwoSortedLists() {
+        SinglyLinkedList listA = new SinglyLinkedList();
+        SinglyLinkedList listB = new SinglyLinkedList();
+
+        for (int i = 2; i <= 10; i += 2) {
+            listA.insert(i);
+            listB.insert(i - 1);
+        }
+
+        SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
+        assertEquals("1->2->3->4->5->6->7->8->9->10", mergedList.toString(), "Merged list should contain all elements in sorted order.");
+    }
+
+    @Test
+    void testMergeWithEmptyListA() {
+        SinglyLinkedList listA = new SinglyLinkedList(); // Empty listA
+        SinglyLinkedList listB = new SinglyLinkedList();
+        listB.insert(1);
+        listB.insert(3);
+        listB.insert(5);
+
+        SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
+        assertEquals("1->3->5", mergedList.toString(), "Merged list should match listB when listA is empty.");
+    }
+
+    @Test
+    void testMergeWithEmptyListB() {
+        SinglyLinkedList listA = new SinglyLinkedList();
+        SinglyLinkedList listB = new SinglyLinkedList(); // Empty listB
+        listA.insert(2);
+        listA.insert(4);
+        listA.insert(6);
+
+        SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
+        assertEquals("2->4->6", mergedList.toString(), "Merged list should match listA when listB is empty.");
+    }
+
+    @Test
+    void testMergeWithBothEmptyLists() {
+        SinglyLinkedList listA = new SinglyLinkedList(); // Empty listA
+        SinglyLinkedList listB = new SinglyLinkedList(); // Empty listB
+
+        SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
+        assertEquals("", mergedList.toString(), "Merged list should be empty when both input lists are empty.");
+    }
+
+    @Test
+    void testMergeWithDuplicateValues() {
+        SinglyLinkedList listA = new SinglyLinkedList();
+        SinglyLinkedList listB = new SinglyLinkedList();
+
+        listA.insert(1);
+        listA.insert(3);
+        listA.insert(5);
+        listB.insert(1);
+        listB.insert(4);
+        listB.insert(5);
+
+        SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
+        assertEquals("1->1->3->4->5->5", mergedList.toString(), "Merged list should include duplicate values in sorted order.");
+    }
+
+    @Test
+    void testMergeThrowsExceptionOnNullInput() {
+        SinglyLinkedList listA = null;
+        SinglyLinkedList listB = new SinglyLinkedList();
+        listB.insert(1);
+        listB.insert(2);
+
+        SinglyLinkedList finalListA = listA;
+        SinglyLinkedList finalListB = listB;
+        assertThrows(NullPointerException.class, () -> MergeSortedSinglyLinkedList.merge(finalListA, finalListB), "Should throw NullPointerException if listA is null.");
+
+        listA = new SinglyLinkedList();
+        listB = null;
+        SinglyLinkedList finalListA1 = listA;
+        SinglyLinkedList finalListB1 = listB;
+        assertThrows(NullPointerException.class, () -> MergeSortedSinglyLinkedList.merge(finalListA1, finalListB1), "Should throw NullPointerException if listB is null.");
+    }
+}