Swap Nodes in Pairs

/*

Swap Nodes in PairsFeb 15 '12

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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(head==NULL) return NULL;
        if(head->next == NULL) return head;
        ListNode *first = head;
        ListNode *second = head->next;
        int temp;
        while(first!=NULL && second!=NULL){
            temp = first->val;
            first->val = second->val;
            second->val = temp;
            first = second->next;
            if(first!=NULL) second = first->next;
        }
       
        return head;
    }
};