[Leetcode]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.

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    /*algorithm        1)three pointer, tail,shead,stail        2)swap shead,stail        3)move tail to shead        time O(n) space O(1)    */    ListNode* swapPairs(ListNode* head) {        ListNode dummy(0),*tail = &dummy;        ListNode*shead,*stail;        dummy.next = head;        while(tail && tail->next&& tail->next->next){            shead= tail->next;            stail = shead->next;            shead->next = stail->next;            stail->next = shead;            tail->next = stail;            tail = shead;        }        return dummy.next;    }};