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.

AC代码：

class Solution(object):
    def swapPairs(self, head):
        if not head: return None
        if not head.next: return head
        ret_node = head.next
        while True:
            temp = head.next.next
            head.next.next = head
            if not temp or not temp.next:
                head.next = temp
                break
            else:
                head.next = temp.next
                head = temp
        return ret_node

规定了不能交换值，只能改变节点。

所以每两个节点分为一组，用一个临时变量存储后面节点即可。