/** * 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* res = head;        ListNode* term = res;        while(head!=NULL)        {            if(head->next!=NULL)            {                ListNode* temp = head->next;                                head->next = temp->next;                temp->next = head;                term->next = temp;                                 term=head;                head=head->next;                          }            else                break;        }        return res;    }};