24. Swap Nodes in Pairs

/**
 * 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* node;
        ListNode* pre;
        ListNode* next;
        ListNode* temp;
        ListNode* result;
        if(head == NULL)
            return NULL;
        if(head->next ==NULL)
            return head;
        node = head;
        next = node->next;
        result = next;
        
        temp=next->next;
        node->next = temp;
        next->next = node;
        pre = node;
        
        
        while((pre->next!=NULL)&&(pre->next->next!=NULL))
        {
            node = pre->next;
            next = node->next;
            
            temp=next->next;
            node->next=temp;
            pre->next = next;
            next->next = node;
            pre=node;
        }
        return result;
    }
};