Add Middle of Linked List (Slow/Fast Pointers) (#7212)

* feat: implement Smooth Sort algorithm with detailed JavaDoc and test class

* style: format LEONARDO array for improved readability with clang-format

* feat: add MiddleOfLinkedList class and corresponding test cases

* docs: update documentation for MiddleOfLinkedList class

* test: refactor MiddleOfLinkedListTest to improve readability and assertions

* test: refactor MiddleOfLinkedListTest for improved null safety and readability

---------

Co-authored-by: Ahmed Allam <60698204+AllamF5J@users.noreply.github.com>

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

@@ -0,0 +1,74 @@
+package com.thealgorithms.datastructures.lists;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import java.util.Objects;
+import org.junit.jupiter.api.Test;
+
+public class MiddleOfLinkedListTest {
+
+    private static SinglyLinkedListNode listOf(int firstValue, int... remainingValues) {
+        SinglyLinkedListNode head = new SinglyLinkedListNode(firstValue);
+        SinglyLinkedListNode current = head;
+
+        for (int i = 0; i < remainingValues.length; i++) {
+            current.next = new SinglyLinkedListNode(remainingValues[i]);
+            current = current.next;
+        }
+        return head;
+    }
+
+    @Test
+    void middleNodeOddLength() {
+        SinglyLinkedListNode head = listOf(1, 2, 3, 4, 5);
+        SinglyLinkedListNode middle = Objects.requireNonNull(MiddleOfLinkedList.middleNode(head));
+        assertEquals(3, middle.value);
+    }
+
+    @Test
+    void middleNodeEvenLengthReturnsSecondMiddle() {
+        SinglyLinkedListNode head = listOf(1, 2, 3, 4, 5, 6);
+        SinglyLinkedListNode middle = Objects.requireNonNull(MiddleOfLinkedList.middleNode(head));
+        assertEquals(4, middle.value);
+    }
+
+    @Test
+    void middleNodeSingleElement() {
+        SinglyLinkedListNode head = listOf(42);
+        SinglyLinkedListNode middle = Objects.requireNonNull(MiddleOfLinkedList.middleNode(head));
+        assertEquals(42, middle.value);
+    }
+
+    @Test
+    void middleNodeTwoElementsReturnsSecond() {
+        SinglyLinkedListNode head = listOf(10, 20);
+        SinglyLinkedListNode middle = Objects.requireNonNull(MiddleOfLinkedList.middleNode(head));
+        assertEquals(20, middle.value);
+    }
+
+    @Test
+    void middleNodeNullHead() {
+        assertNull(MiddleOfLinkedList.middleNode(null));
+    }
+
+    @Test
+    void middleNodeDoesNotModifyListStructure() {
+        SinglyLinkedListNode first = new SinglyLinkedListNode(1);
+        SinglyLinkedListNode second = new SinglyLinkedListNode(2);
+        SinglyLinkedListNode third = new SinglyLinkedListNode(3);
+        SinglyLinkedListNode fourth = new SinglyLinkedListNode(4);
+
+        first.next = second;
+        second.next = third;
+        third.next = fourth;
+
+        SinglyLinkedListNode middle = Objects.requireNonNull(MiddleOfLinkedList.middleNode(first));
+        assertEquals(3, middle.value);
+
+        assertEquals(second, first.next);
+        assertEquals(third, second.next);
+        assertEquals(fourth, third.next);
+        assertNull(fourth.next);
+    }
+}