24. 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.

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

class Solution:
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        start = ListNode(0)
        start.next = head
        headpre = start
        while headpre.next and headpre.next.next:
            # print('headpre:',headpre.val,'head:',head.val)
            temp = headpre.next.next
            headpre.next.next = temp.next
            temp.next = headpre.next
            headpre.next = temp
            headpre = temp.next
            # print('headpre:',headpre.val,'head:',head.val)
        return start.next