public ListNode swapPairs(ListNode head) {
if (head == null) return null;
if (head.next == null) return head;
ListNode newHead = head.next;
head.next = swapPairs(newHead.next);
newHead.next = head;
return newHead;
}
public ListNode createList(int[] intArray, int linkIndex) {
ListNode head = new ListNode(intArray[0]);
ListNode previousNode = head;
ListNode linkedBackNode = null;
for (int i = 1; i < intArray.length; i++) {
ListNode node = new ListNode(intArray[i]);
if (linkIndex == i) linkedBackNode = node;
previousNode.next = node;
previousNode = node;
}
previousNode.next = linkedBackNode;
return head;
}
public ListNode deleteDuplicates(ListNode head) {
if (head == null) return null;
if (head.next == null) return head;
head.next = deleteDuplicates(head.next);
if (head.val == head.next.val) return head.next;
else return head;
}