[leetcode]Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

• Your algorithm should use only constant extra space.

• You may not modify the values in the list's nodes, only nodes itself may be changed.

思路：有点绕的时候，画个图就好喽

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

class Solution(object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        res = pre = ListNode(0)
        pre.next = head
        while pre.next and pre.next.next:
            a = pre.next
            b = pre.next.next
            pre.next, b.next, a.next = b, a, b.next
            pre = a
        return res.next