24. Swap Nodes in Pairs

 1 class Solution {
 2    public ListNode swapPairs(ListNode head) {
 3        if(head == null) return null;
 4        if(head.next == null) return head;
 5        ListNode node2 = head;
 6        ListNode node3 = head.next;
 7        if(node3.next == null) {
 8            node2.next = null;
 9            node3.next = node2;
10            return node3;
11        }
12        node2.next = node3.next;
13        node3.next = node2;
14        head = node3;
15        ListNode prevNode = node2;
16        while(node2.next != null && node2.next.next != null) {
17            node2 = node2.next;
18            node3 = node2.next;
19            prevNode.next = node3;
20            node2.next = node3.next;
21            node3.next = node2;
22            prevNode = node2;
23               
24        }
25      return head;   
26    }
27 }