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/main/java/com/thealgorithms/datastructures/lists/MiddleOfLinkedList.java

@@ -0,0 +1,46 @@
+package com.thealgorithms.datastructures.lists;
+
+/**
+ * Returns the middle node of a singly linked list using the two-pointer technique.
+ *
+ * <p>The {@code slow} pointer advances by one node per iteration while {@code fast} advances by two.
+ * When {@code fast == null} or {@code fast.next == null}, {@code slow} points to the middle node.
+ * For even-length lists, this returns the <em>second</em> middle node.</p>
+ *
+ * <p>This method does not modify the input list.</p>
+ *
+ * <p>Reference: https://en.wikipedia.org/wiki/Cycle_detection#Floyd's_tortoise_and_hare</p>
+ *
+ * <p>Complexity:</p>
+ * <ul>
+ *   <li>Time: {@code O(n)}</li>
+ *   <li>Space: {@code O(1)}</li>
+ * </ul>
+ */
+public final class MiddleOfLinkedList {
+
+    private MiddleOfLinkedList() {
+    }
+
+    /**
+     * Returns the middle node of the list.
+     *
+     * @param head the head of the singly linked list; may be {@code null}
+     * @return the middle node (second middle for even-sized lists), or {@code null} if {@code head} is {@code null}
+     */
+    public static SinglyLinkedListNode middleNode(final SinglyLinkedListNode head) {
+        if (head == null) {
+            return null;
+        }
+
+        SinglyLinkedListNode slow = head;
+        SinglyLinkedListNode fast = head;
+
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+
+        return slow;
+    }
+}

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);
+    }
+}