24. Swap Nodes in Pairs

预处理:在head之前添加一个缓冲节点，最后return head->next

pre---A--B--last变为：pre--B--A--last

/**
 * 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 *Newhead=(ListNode*)malloc(sizeof(ListNode));
        Newhead->val=0;
        Newhead->next=head;

        ListNode *pre = Newhead;
        ListNode *A=head,*B=head->next;
        ListNode *last=B->next;

        while(A!=NULL&&B!=NULL)
        {
             pre->next = B;
             B->next =A;
             A->next = last;

             pre = A;
             A=pre->next;
            if(A==NULL)
 		 	  break;
             B=A->next;
            if(B==NULL)
 		 	  break;
             last=B->next;
        }
        return Newhead->next;

    }
};