24. Swap Nodes in Pairs

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

For example,
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.

Python:

# 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
"""
if head == None or head.next == None:
return head
else:
pre = ListNode(0)
pre.next = head
cur = pre
while cur.next and cur.next.next:
tmp = cur.next.next
cur.next.next = tmp.next
tmp.next = cur.next
cur.next = tmp
cur = cur.next.next
return pre.next