24. Swap Nodes in Pairs

24. 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.

找时间加入递归的写法 

class Solution(object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head==None or head.next==None:
            return head
        temp_head=ListNode(-1)
        temp_head.next=head
        prehead=temp_head
        while prehead.next!=None and prehead.next.next!=None:
            a=prehead.next
            b=a.next
            prehead.next,a.next,b.next=b,b.next,a
            prehead=a
        return temp_head.next