[FEAT] Implement Flattening MultiLevel LinkedList (#6670)

[FEAT] Implement Flattening Multilevel LinkedList

Co-authored-by: Priyanshu1303d <priyanshu130d@gmail.com>

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

@@ -0,0 +1,112 @@
+package com.thealgorithms.datastructures.lists;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import java.util.ArrayList;
+import java.util.List;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+/**
+ * Unit tests for the FlattenMultilevelLinkedList class.
+ * This class tests the flattening logic with various list structures,
+ * including null lists, simple lists, and complex multilevel lists.
+ */
+final class FlattenMultilevelLinkedListTest {
+
+    // A helper function to convert a flattened list (connected by child pointers)
+    // into a standard Java List for easy comparison.
+    private List<Integer> toList(FlattenMultilevelLinkedList.Node head) {
+        List<Integer> list = new ArrayList<>();
+        FlattenMultilevelLinkedList.Node current = head;
+        while (current != null) {
+            list.add(current.data);
+            current = current.child;
+        }
+        return list;
+    }
+
+    @Test
+    @DisplayName("Test with a null list")
+    void testFlattenNullList() {
+        assertNull(FlattenMultilevelLinkedList.flatten(null));
+    }
+
+    @Test
+    @DisplayName("Test with a simple, single-level list")
+    void testFlattenSingleLevelList() {
+        // Create a simple list: 1 -> 2 -> 3
+        FlattenMultilevelLinkedList.Node head = new FlattenMultilevelLinkedList.Node(1);
+        head.next = new FlattenMultilevelLinkedList.Node(2);
+        head.next.next = new FlattenMultilevelLinkedList.Node(3);
+
+        // Flatten the list
+        FlattenMultilevelLinkedList.Node flattenedHead = FlattenMultilevelLinkedList.flatten(head);
+
+        // Expected output: 1 -> 2 -> 3 (vertically)
+        List<Integer> expected = List.of(1, 2, 3);
+        assertEquals(expected, toList(flattenedHead));
+    }
+
+    @Test
+    @DisplayName("Test with a complex multilevel list")
+    void testFlattenComplexMultilevelList() {
+        // Create the multilevel structure from the problem description
+        // 5 -> 10 -> 19 -> 28
+        // |    |     |     |
+        // 7    20    22    35
+        // |          |     |
+        // 8          50    40
+        // |                |
+        // 30               45
+        FlattenMultilevelLinkedList.Node head = new FlattenMultilevelLinkedList.Node(5);
+        head.child = new FlattenMultilevelLinkedList.Node(7);
+        head.child.child = new FlattenMultilevelLinkedList.Node(8);
+        head.child.child.child = new FlattenMultilevelLinkedList.Node(30);
+
+        head.next = new FlattenMultilevelLinkedList.Node(10);
+        head.next.child = new FlattenMultilevelLinkedList.Node(20);
+
+        head.next.next = new FlattenMultilevelLinkedList.Node(19);
+        head.next.next.child = new FlattenMultilevelLinkedList.Node(22);
+        head.next.next.child.child = new FlattenMultilevelLinkedList.Node(50);
+
+        head.next.next.next = new FlattenMultilevelLinkedList.Node(28);
+        head.next.next.next.child = new FlattenMultilevelLinkedList.Node(35);
+        head.next.next.next.child.child = new FlattenMultilevelLinkedList.Node(40);
+        head.next.next.next.child.child.child = new FlattenMultilevelLinkedList.Node(45);
+
+        // Flatten the list
+        FlattenMultilevelLinkedList.Node flattenedHead = FlattenMultilevelLinkedList.flatten(head);
+
+        // Expected sorted output
+        List<Integer> expected = List.of(5, 7, 8, 10, 19, 20, 22, 28, 30, 35, 40, 45, 50);
+        assertEquals(expected, toList(flattenedHead));
+    }
+
+    @Test
+    @DisplayName("Test with some empty child lists")
+    void testFlattenWithEmptyChildLists() {
+        // Create a list: 5 -> 10 -> 12
+        // |           |
+        // 7           11
+        // |
+        // 9
+        FlattenMultilevelLinkedList.Node head = new FlattenMultilevelLinkedList.Node(5);
+        head.child = new FlattenMultilevelLinkedList.Node(7);
+        head.child.child = new FlattenMultilevelLinkedList.Node(9);
+
+        head.next = new FlattenMultilevelLinkedList.Node(10); // No child list
+        head.next.child = null;
+
+        head.next.next = new FlattenMultilevelLinkedList.Node(12);
+        head.next.next.child = new FlattenMultilevelLinkedList.Node(16);
+
+        // Flatten the list
+        FlattenMultilevelLinkedList.Node flattenedHead = FlattenMultilevelLinkedList.flatten(head);
+
+        // Expected sorted output
+        List<Integer> expected = List.of(5, 7, 9, 10, 12, 16);
+        assertEquals(expected, toList(flattenedHead));
+    }
+}