feat: add MiddleOfLinkedList class and corresponding test cases

Author: GziXnine

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

@@ -0,0 +1,44 @@
+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>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 +1,71 @@
+package com.thealgorithms.datastructures.lists;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import org.junit.jupiter.api.Test;
+
+public class MiddleOfLinkedListTest {
+
+    private static SinglyLinkedListNode listOf(int... values) {
+        if (values == null || values.length == 0) {
+            return null;
+        }
+
+        SinglyLinkedListNode head = new SinglyLinkedListNode(values[0]);
+        SinglyLinkedListNode current = head;
+        for (int i = 1; i < values.length; i++) {
+            current.next = new SinglyLinkedListNode(values[i]);
+            current = current.next;
+        }
+        return head;
+    }
+
+    @Test
+    void middleNodeOddLength() {
+        SinglyLinkedListNode head = listOf(1, 2, 3, 4, 5);
+        assertEquals(3, MiddleOfLinkedList.middleNode(head).value);
+    }
+
+    @Test
+    void middleNodeEvenLengthReturnsSecondMiddle() {
+        SinglyLinkedListNode head = listOf(1, 2, 3, 4, 5, 6);
+        assertEquals(4, MiddleOfLinkedList.middleNode(head).value);
+    }
+
+    @Test
+    void middleNodeSingleElement() {
+        SinglyLinkedListNode head = listOf(42);
+        assertEquals(42, MiddleOfLinkedList.middleNode(head).value);
+    }
+
+    @Test
+    void middleNodeTwoElementsReturnsSecond() {
+        SinglyLinkedListNode head = listOf(10, 20);
+        assertEquals(20, MiddleOfLinkedList.middleNode(head).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;
+
+        assertEquals(3, MiddleOfLinkedList.middleNode(first).value);
+
+        assertEquals(second, first.next);
+        assertEquals(third, second.next);
+        assertEquals(fourth, third.next);
+        assertNull(fourth.next);
+    }
+}