[24] Swap Nodes in Pairs

https://leetcode.com/problems/swap-nodes-in-pairs/description/

• algorithms
• Medium (41.72%)
• Source Code: 24.swap-nodes-in-pairs.py
• Total Accepted: 300.9K
• Total Submissions: 682.9K
• Testcase Example: '[1,2,3,4]'

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

You may not modify the values in the list's nodes, only nodes itself may be changed.

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

class Solution(object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """