prev of old head needs to refer to the new head

src/main/java/io/ad/structs/DoubleLinkedList.java

@@ -43,6 +43,7 @@ public class DoubleLinkedList<E> implements List<E> {
 		}
 		if (index == 0) {
 			ListNode newHead = new ListNode(element, null, head);
+			head.prev=newHead;
 			head = newHead;
 			if (size()==0) last=newHead;
 			size++;