LintCode Python 简单级题目 451.两两交换链表中的节点

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param head, a ListNode
    # @return a ListNode
    def swapPairs(self, head):
        # Write your code here
        new = ListNode(0)
        new.next = head
        head = new
        
        while head.next is not None and head.next.next is not None:
            n1 = head.next
            n2 = head.next.next
            # 交换n1、n2
            head.next = n2
            n1.next = n2.next
            n2.next = n1
            # 交换后的链表，n1在n2后面，将head指向n1
            head = n1
        
        return new.next
        
    # 不创建链表头是否可行？lintcode报超时。
    def _swapPairs(self, head):
        # Write your code here
        if head is None or head.next is None: 
            return head
            
        new = head
        n1 = head
        n2 = head.next
        while n1.next is not None and n2.next is not None:
            n1.next = n2.next
            n2.next = n1
            n1 = n1.next
        return new