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:    ListNode* swapPairs(ListNode* head) {        ListNode*p=head;        if(p==NULL||p->next==NULL)return head;        head=p->next;        ListNode*q=head->next;        head->next=p;        p->next=q;        while(q!=NULL)        {            if(q->next!=NULL)            {                ListNode*t=q->next->next;                p->next=q->next;                p->next->next=q;                q->next=t;                p=p->next->next;                q=t;            }            else break;        }        return head;    }};