leetcode[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) {
    if(head==NULL||head->next==NULL)return head;
    ListNode *res=new ListNode(-1);
    res->next=head;
    ListNode *front=head->next;
    ListNode *later=head;
    ListNode *p=res;
    while(front!=NULL)
    {
        later->next=front->next;
        front->next=later;
        p->next=front;
        p=later;
        if (later->next==NULL)break;
        later=later->next;
        front=later->next;
    }
    return res->next;
    }
};