Leetcode-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

/**
 * 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;
        ListNode* q=p;
        while(p!=NULL&&p->next!=NULL)
        {
            ListNode* newlist=new ListNode(p->val);
            p->val=p->next->val;
            newlist->next=p->next->next;
            p->next=newlist;
            p=p->next->next;
        }
        return q;
    }
};