leetcode24-swap nodes in pairs

class (object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        res = ListNode(0)
        res.next = head
        cur = res
        while cur.next and cur.next.next:
            a = cur.next
            b = cur.next.next
            cur.next, b.next, a.next = b, a, b.next
            
            # cur.next = b
            # b.next = a
            # a.next = c
            cur = a
        return res.next