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.

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/**

 * 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)  
            return NULL;  
        ListNode *addHead = new ListNode(-1);  
        addHead->next = head;  

        ListNode *first = head, *second = head->next,*preSwap = addHead;  

        int a;

        int b;

        while (second)  
        {  
            preSwap->next = second;  
            first->next = second->next;  
            second->next = first;  
            preSwap = first;  
            first = first->next;  
            if (!first)  
                break;  
            second = first->next;  
        }  
        return addHead->next;  
    }
};