5/30/2017 - 2:28 PM

## 24. Swap Nodes in Pairs

1. Swap Nodes in Pairs
``````# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
"""
:rtype: ListNode
"""
dummy = ListNode(0)
return dummy.next
def recursive(self, node):
if node == None or node.next == None:
return node
p = node.next
node.next = self.recursive(p.next)
p.next = node
return p``````
``````
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
"""
:rtype: ListNode
"""
dummy = ListNode(0)
prev = dummy
p = dummy.next
while p != None and p.next != None:
n = p.next
nn = n.next
prev.next = n
n.next = p
p.next = nn
prev = p
p = nn
return dummy.next``````

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.