24. Swap Nodes in Pairs

题目：

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.

链接： http://leetcode.com/problems/swap-nodes-in-pairs/

一刷

class Solution(object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        dummy_node = ListNode(0)
        dummy_node.next = head
        prev = dummy_node
        cur = head

        while cur and cur.next:
            prev.next = cur.next
            cur.next = cur.next.next
            prev.next.next = cur

            prev = cur
            cur = cur.next
        return dummy_node.next

二刷需要加上recursion解法