# Definition for singly-linked list.

class ListNode(object):

def __init__(self, val=0, next=None):

self.val = val

self.next = next

def create_link(nums):

head = ListNode(None)

cur_node = head

for i in range(len(nums)):

cur_node.next = ListNode(nums[i])

cur_node = cur_node.next

return head.next

def check_link(link):

while link != None:

print(link.val, end=' ')

link = link.next

print()

class Solution(object):

def swapPairs(self, head):

"""

:type head: ListNode

:rtype: ListNode

"""

if head == None: return None

if head.next == None: return head

if head.next.next == None:

new_head = head.next

new_head.next = head

new_head.next.next = None

return new_head

new_head = ListNode(0)

new_head.next = head

font_point = rear_point = new_head

for i in range(3): font_point = font_point.next

while font_point != None:

tmp = rear_point.next

rear_point.next = rear_point.next.next

rear_point.next.next = tmp

tmp.next = font_point

rear_point = rear_point.next.next

if font_point.next != None:

if font_point.next.next != None:

font_point = font_point.next.next

else:

rear_point.next = rear_point.next.next

rear_point.next.next = font_point

font_point.next = None

return new_head.next

else:

return new_head.next

if __name__ == '__main__':

l = create_link([1, 2, 3, 4])

s = Solution()

result = s.swapPairs(l)

check_link(result)