1 """
2 Given a linked list, swap every two adjacent nodes and return its head.
3 You may not modify the values in the list's nodes, only nodes itself may be changed.
4 Example:
5 Given 1->2->3->4, you should return the list as 2->1->4->3.
6 """
7 class ListNode:
8 def __init__(self, x):
9 self.val = x
10 self.next = None
11
12 class Solution:
13 def swapPairs(self, head):
14 first = ListNode(0)
15 first.next = head
16 if head == None or head.next == None:
17 return head
18 p = first #三个指针,p代表previous前结点,m代表medium中结点,l代表last后结点
19 m = head
20 l = head.next
21 while m and l: #bug只写了 while l:
22 m.next = l.next #因为下面是if句，此处还应再判断一下m! = None
23 l.next = m
24 p.next = l
25 p = m
26 m = p.next
27 if m != None: #BUG 没写if判断句，首先要保证m != None，才能保证m.next不为None
28 l = m.next
29 return first.next
