Enhance docs, add tests in MergeKSortedLinkedList (#5995)

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

@@ -0,0 +1,93 @@
+package com.thealgorithms.datastructures.lists;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import com.thealgorithms.datastructures.lists.MergeKSortedLinkedList.Node;
+import java.util.Arrays;
+import org.junit.jupiter.api.Test;
+
+class MergeKSortedLinkedListTest {
+
+    @Test
+    void testMergeKLists() {
+        Node list1 = new Node(1, new Node(4, new Node(5)));
+        Node list2 = new Node(1, new Node(3, new Node(4)));
+        Node list3 = new Node(2, new Node(6));
+        Node[] lists = {list1, list2, list3};
+
+        MergeKSortedLinkedList merger = new MergeKSortedLinkedList();
+        Node mergedHead = merger.mergeKList(lists, lists.length);
+
+        int[] expectedValues = {1, 1, 2, 3, 4, 4, 5, 6};
+        int[] actualValues = getListValues(mergedHead);
+        assertArrayEquals(expectedValues, actualValues, "Merged list values do not match the expected sorted order.");
+    }
+
+    @Test
+    void testMergeEmptyLists() {
+        Node[] lists = {null, null, null};
+
+        MergeKSortedLinkedList merger = new MergeKSortedLinkedList();
+        Node mergedHead = merger.mergeKList(lists, lists.length);
+
+        assertNull(mergedHead, "Merged list should be null when all input lists are empty.");
+    }
+
+    @Test
+    void testMergeSingleList() {
+        Node list1 = new Node(1, new Node(3, new Node(5)));
+        Node[] lists = {list1};
+
+        MergeKSortedLinkedList merger = new MergeKSortedLinkedList();
+        Node mergedHead = merger.mergeKList(lists, lists.length);
+
+        int[] expectedValues = {1, 3, 5};
+        int[] actualValues = getListValues(mergedHead);
+        assertArrayEquals(expectedValues, actualValues, "Merged list should match the single input list when only one list is provided.");
+    }
+
+    @Test
+    void testMergeListsOfDifferentLengths() {
+        Node list1 = new Node(1, new Node(3, new Node(5)));
+        Node list2 = new Node(2, new Node(4));
+        Node list3 = new Node(6);
+        Node[] lists = {list1, list2, list3};
+
+        MergeKSortedLinkedList merger = new MergeKSortedLinkedList();
+        Node mergedHead = merger.mergeKList(lists, lists.length);
+
+        int[] expectedValues = {1, 2, 3, 4, 5, 6};
+        int[] actualValues = getListValues(mergedHead);
+        assertArrayEquals(expectedValues, actualValues, "Merged list values do not match expected sorted order for lists of different lengths.");
+    }
+
+    @Test
+    void testMergeSingleElementLists() {
+        Node list1 = new Node(1);
+        Node list2 = new Node(3);
+        Node list3 = new Node(2);
+        Node[] lists = {list1, list2, list3};
+
+        MergeKSortedLinkedList merger = new MergeKSortedLinkedList();
+        Node mergedHead = merger.mergeKList(lists, lists.length);
+
+        int[] expectedValues = {1, 2, 3};
+        int[] actualValues = getListValues(mergedHead);
+        assertArrayEquals(expectedValues, actualValues, "Merged list values do not match expected sorted order for single-element lists.");
+    }
+
+    /**
+     * Helper method to extract values from the linked list into an array for assertion.
+     */
+    private int[] getListValues(Node head) {
+        int[] values = new int[100]; // assuming max length for simplicity
+        int i = 0;
+        Node curr = head;
+        while (curr != null) {
+            values[i++] = curr.data;
+            curr = curr.next;
+        }
+        return Arrays.copyOf(values, i); // return only filled part
+    }
+}