```
/**
* 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;
if (!head->next) return head;
ListNode* newHead = head->next;
ListNode* first = head;
ListNode* second = head->next;
ListNode* hold = NULL;
while (true){
//swap
first->next = second->next;
second->next = first;
if (hold != NULL) hold->next = second;
//increment
hold = first;
if (first->next && first->next->next){
first = first->next;
second = first->next;
}
else{
break;
}
}
return newHead;
}
};
```