Swap Nodes in Pairs

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode swapPairs(ListNode head) {
ListNode dummy = new ListNode(0);
ListNode prev = dummy;
ListNode node = head;
dummy.next = node;
int count = 0;
while (node != null) {
ListNode next = node.next;
count++;
if (count == 2) {
prev = helper(prev, next);
count = 0;
}
node = next;
}
return dummy.next;
}

private ListNode helper(ListNode prev, ListNode next) {
ListNode last = prev.next;
ListNode cur = last.next;
while (cur != next) {
last.next = cur.next;
cur.next = prev.next;
prev.next = cur;
cur = last.next;
}
return last;
}

// public ListNode swapPairs(ListNode head) {
//     ListNode dummy = new ListNode(0);
//     ListNode prev = dummy;
//     dummy.next = head;
//     while (head != null && head.next != null) {
//         ListNode next = head.next.next;
//         prev.next = head.next;
//         head.next.next = head;
//         prev = head;
//         head.next = next;
//         head = next;
//     }
//     return dummy.next;
// }
}