24.py

'''
 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.
'''

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

classSolution(object):
defswapPairs(self, head):
"""
 :type head: ListNode
 :rtype: ListNode
 """

ifheadisNone:
returnhead

ref=head

whilerefisnotNoneandref.nextisnotNone:
ref.val, ref.next.val=ref.next.val, ref.val
ref=ref.next.next

returnhead