6 链表成对调换

1->2->3->4转换成2->1->4->3.

class ListNode:
  def __init__(self, x):
    self.val = x
    self.next = None

class Solution:
  # @param a ListNode
  # @return a ListNode
  def swapPairs(self, head):
    if head != None and head.next != None:
      next = head.next
      head.next = self.swapPairs(next.next)
      next.next = head
      return next
    return head