24.py

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

	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
        """
        
        if head is None:
            return head
        
        ref = head
        
        while ref is not None and ref.next is not None:
            ref.val, ref.next.val = ref.next.val, ref.val
            ref = ref.next.next
            
        return head