链表的两两交换！

"""
Definition of ListNode
class ListNode(object):
    def __init__(self, val, next=None):
        self.val = val
        self.next = next
"""

class Solution:
    """
    @param head: a ListNode
    @return: a ListNode
    """
    def swapPairs(self, head):
        # write your code here
        if head == None:
            return 
        cur = ListNode(0)
        cur.next = head
        
        first = cur #first始终没变 指向head  故返回其next就是整个链表
        #整个循环就是 temp的使用
        while cur.next and cur.next.next:
            cur1 = cur.next
            cur2 = cur1.next
            cur3 = cur2.next
            cur1.next = cur3
            cur2.next = cur1
            cur.next = cur2
            
            cur = cur1
            
        return first.next