Given a linked list, swap every two adjacent nodes and return its head.
You may not modify the values in the list's nodes, only nodes itself may be changed.
双指针，q 在前，p 在后，交换 p、q；然后 p、 q 一起向后走两步，交换 p、q；重复操作至到 q 走到链表末尾。
# Definition for singly-linked list.
class ListNode:
def __init__(self, x):
self.val = x
self.next = None
class Solution:
def swapPairs(self, head: ListNode) -> ListNode:
if not head or not head.next:
return head
dummy = ListNode(-1)
dummy.next = head
o, p, q = dummy, head, head.next
while True:
# swap
o.next = q
p.next = q.next
q.next = p
p, q = q, p
# forward
if q.next and q.next.next:
o = o.next.next
p = p.next.next
q = q.next.next
else:
break
return dummy.next