Added FindMiddleNode, FindMiddleNodeTest, ListNode, and corresponding…

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

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+package com.thealgorithms.datastructures.lists;
+
+public final class FindMiddleNode {
+
+    private FindMiddleNode() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    public static ListNode findMiddle(ListNode head) {
+        ListNode slow = head;
+        ListNode fast = head;
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+        return slow;
+    }
+
+    public static void main(String[] args) {
+        // Example: create a linked list 1->2->3->4->5
+        ListNode head = new ListNode(1);
+        head.next = new ListNode(2);
+        head.next.next = new ListNode(3);
+        head.next.next.next = new ListNode(4);
+        head.next.next.next.next = new ListNode(5);
+
+        ListNode middle = findMiddle(head);
+        System.out.println("Middle node value: " + middle.value);
+    }
+}

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

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+package com.thealgorithms.datastructures.lists;
+
+public class ListNode {
+    int value;
+    ListNode next;
+
+    public ListNode(int value) {
+        this.value = value;
+        this.next = null;
+    }
+}

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

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+package com.thealgorithms.datastructures.lists;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class FindMiddleNodeTest {
+
+    @Test
+    public void testOddLengthList() {
+        ListNode head = createList(new int[] { 1, 2, 3, 4, 5 });
+        assertEquals(3, FindMiddleNode.findMiddle(head).value);
+    }
+
+    @Test
+    public void testEvenLengthList() {
+        ListNode head = createList(new int[] { 1, 2, 3, 4, 5, 6 });
+        assertEquals(4, FindMiddleNode.findMiddle(head).value);
+    }
+
+    private ListNode createList(int[] values) {
+        ListNode head = new ListNode(values[0]);
+        ListNode current = head;
+        for (int i = 1; i < values.length; i++) {
+            current.next = new ListNode(values[i]);
+            current = current.next;
+        }
+        return head;
+    }
+}