Added Single Linked List, Doubly Linked List, and Tortoise and Hare algorithms

Author: Chintha-vardhan

src/main/java/com/thealgorithms/LinkedList/DoublyLinkedLIst.java

@@ -0,0 +1,29 @@
+public class SinglyLinkedList {
+    class Node {
+        int data;
+        Node next;
+        Node(int d) { data = d; next = null; }
+    }
+
+    private Node head;
+
+    public void insertAtEnd(int data) {
+        Node newNode = new Node(data);
+        if (head == null) {
+            head = newNode;
+            return;
+        }
+        Node curr = head;
+        while (curr.next != null) curr = curr.next;
+        curr.next = newNode;
+    }
+
+    public void display() {
+        Node curr = head;
+        while (curr != null) {
+            System.out.print(curr.data + " -> ");
+            curr = curr.next;
+        }
+        System.out.println("null");
+    }
+}