Leetcode (Python): 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.

Solution:

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

class Solution:
    # @param a ListNode
    # @return a ListNode
    def swapPairs(self, head):
        dummy = ListNode(0)
        dummy.next = head
        
        previous = dummy
        while previous.next != None and previous.next.next:
            node1 = previous.next
            node2 = previous.next.next
            previous.next = node2
            node1.next = node2.next
            node2.next = node1
            previous = node1
        
        return dummy.next