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;       } };