q24_Swap_Pairs.py

# 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 and head.next:
            tmp = head.next
            head.next = self.swapPairs(tmp.next)
            tmp.next = head
            return tmp
        return head