LeetCode: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.

/**
* 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 dummy(-1);

ListNode *pre=&dummy;
ListNode *cur=head;
ListNode *next=cur->next;

while(cur!=NULL&&next!=NULL)
{
pre->next=next;
cur->next=next->next;
next->next=cur;

pre=pre->next->next;
if(cur->next!=NULL)
{
cur=cur->next;
next=cur->next;
}
else
break;
}

return dummy.next;

}
};