Add TortoiseHareAlgo implementation with append, toString, and getMiddle methods

src/main/java/com/thealgorithms/datastructures/lists/README.md

@@ -30,3 +30,4 @@ The `next` variable points to the next node in the data structure and value stor
 6. `MergeKSortedLinkedlist.java` : Merges K sorted linked list with mergesort (mergesort is also the most efficient sorting algorithm for linked list).
 7. `RandomNode.java` : Selects a random node from given linked list and diplays it.
 8. `SkipList.java` : Data Structure used for storing a sorted list of elements with help of a Linked list hierarchy that connects to subsequences of elements.
+9. `TortoiseHareAlgo.java` : Finds the middle element of a linked list using the fast and slow pointer (Tortoise-Hare) algorithm.

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

@@ -0,0 +1,63 @@
+package com.thealgorithms.datastructures.lists;
+
+public class TortoiseHareAlgo<E> {
+    static final class Node<E> {
+        Node<E> next;
+        E value;
+
+        private Node(E value, Node<E> next) {
+            this.value = value;
+            this.next = next;
+        }
+    }
+
+    private Node<E> head = null;
+
+    public TortoiseHareAlgo() {
+        head = null;
+    }
+
+    public void append(E value) {
+        Node<E> newNode = new Node<>(value, null);
+        if (head == null) {
+            head = newNode;
+            return;
+        }
+        Node<E> current = head;
+        while (current.next != null) {
+            current = current.next;
+        }
+        current.next = newNode;
+    }
+
+    public E getMiddle() {
+        if (head == null) {
+            return null;
+        }
+
+        Node<E> slow = head;
+        Node<E> fast = head;
+
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+
+        return slow.value;
+    }
+
+    @Override
+    public String toString() {
+        StringBuilder sb = new StringBuilder("[");
+        Node<E> current = head;
+        while (current != null) {
+            sb.append(current.value);
+            if (current.next != null) {
+                sb.append(", ");
+            }
+            current = current.next;
+        }
+        sb.append("]");
+        return sb.toString();
+    }
+}

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

@@ -0,0 +1,46 @@
+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;
+
+class TortoiseHareAlgoTest {
+
+    @Test
+    void testAppendAndToString() {
+        TortoiseHareAlgo<Integer> list = new TortoiseHareAlgo<>();
+        list.append(10);
+        list.append(20);
+        list.append(30);
+        assertEquals("[10, 20, 30]", list.toString());
+    }
+
+    @Test
+    void testGetMiddleOdd() {
+        TortoiseHareAlgo<Integer> list = new TortoiseHareAlgo<>();
+        list.append(1);
+        list.append(2);
+        list.append(3);
+        list.append(4);
+        list.append(5);
+        assertEquals(3, list.getMiddle());
+    }
+
+    @Test
+    void testGetMiddleEven() {
+        TortoiseHareAlgo<Integer> list = new TortoiseHareAlgo<>();
+        list.append(1);
+        list.append(2);
+        list.append(3);
+        list.append(4);
+        assertEquals(3, list.getMiddle()); // returns second middle
+    }
+
+    @Test
+    void testEmptyList() {
+        TortoiseHareAlgo<Integer> list = new TortoiseHareAlgo<>();
+        assertNull(list.getMiddle());
+        assertEquals("[]", list.toString());
+    }
+}