题目要求：
Given a linked list, swap every two adjacent nodes and return its head.
Examples:
Given 1->2->3->4, you should return the list as 2->1->4->3.
Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.
解题思路：
- 画图解题
# Time: O(n)
# Space: O(1)
class ListNode:
def __init__(self, x):
self.val = x
self.next = None
def __repr__(self):
if self:
return "{} -> {}".format(self.val, self.next)
class Solution:
# @param a ListNode
# @return a ListNode
def swapPairs(self, head):
dummy = ListNode(0)
dummy.next = head
current = dummy
while current.next and current.next.next:
next_one, next_two, next_three = current.next, current.next.next, current.next.next.next
current.next = next_two
next_two.next = next_one
next_one.next = next_three
current = next_one
return dummy.next
