Add algorithm to find middle of singly linked list with tests

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

@@ -0,0 +1,39 @@
+package com.thealgorithms.datastructures.lists;
+
+public final class FindMiddleOfSinglyLinkedList {
+
+    static final class Node<E> {
+        public Node<E> next;
+        public E value;
+
+        Node(E value, Node<E> next) {
+            this.value = value;
+            this.next = next;
+        }
+    }
+
+    /**
+     * Finds the middle node of a singly linked list.
+     *
+     * @param head the head node of the singly linked list
+     * @param <E>  the type of elements in the linked list
+     * @return the middle node of the linked list; if the list is empty, returns null
+     */
+    public static <E> Node<E> findMiddle(Node<E> head) {
+        if (head == null) {
+            return null; // Empty list
+        }
+
+        Node<E> slowPointer = head;
+        Node<E> fastPointer = head;
+
+        // Move fastPointer two steps and slowPointer one step at a time
+        while (fastPointer != null && fastPointer.next != null) {
+            slowPointer = slowPointer.next;
+            fastPointer = fastPointer.next.next;
+        }
+
+        // When fastPointer reaches the end, slowPointer will be at the middle
+        return slowPointer;
+    }
+}

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

@@ -0,0 +1,49 @@
+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;
+
+final class FindMiddleOfSinglyLinkedListTest {
+
+    @Test
+    void testFindMiddleEmptyList() {
+        FindMiddleOfSinglyLinkedList.Node<Integer> head = null;
+        var middle = FindMiddleOfSinglyLinkedList.findMiddle(head);
+        assertNull(middle, "Middle of an empty list should be null.");
+    }
+
+    @Test
+    void testFindMiddleSingleElement() {
+        var head = new FindMiddleOfSinglyLinkedList.Node<>(1, null);
+        var middle = FindMiddleOfSinglyLinkedList.findMiddle(head);
+        assertEquals(1, middle.value,
+            "Middle of a single-element list should be the only element.");
+    }
+
+    @Test
+    void testFindMiddleOddElements() {
+        var head = new FindMiddleOfSinglyLinkedList.Node<>(1,
+            new FindMiddleOfSinglyLinkedList.Node<>(2,
+                new FindMiddleOfSinglyLinkedList.Node<>(3,
+                    new FindMiddleOfSinglyLinkedList.Node<>(4,
+                        new FindMiddleOfSinglyLinkedList.Node<>(5, null)))));
+
+        var middle = FindMiddleOfSinglyLinkedList.findMiddle(head);
+        assertEquals(3, middle.value,
+            "Middle of a 5-element list should be the 3rd element.");
+    }
+
+    @Test
+    void testFindMiddleEvenElements() {
+        var head = new FindMiddleOfSinglyLinkedList.Node<>(1,
+            new FindMiddleOfSinglyLinkedList.Node<>(2,
+                new FindMiddleOfSinglyLinkedList.Node<>(3,
+                    new FindMiddleOfSinglyLinkedList.Node<>(4, null))));
+
+        var middle = FindMiddleOfSinglyLinkedList.findMiddle(head);
+        assertEquals(3, middle.value,
+            "Middle of a 4-element list should be the 3rd element.");
+    }
+}