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)
return head;
ListNode* dummy=new ListNode(-),*pre=dummy;
dummy->next=head;
while(pre->next&&pre->next->next)
{
ListNode* last=pre->next->next;
pre->next->next=last->next;
last->next=pre->next;
pre->next=last;
pre=last->next;
}
return dummy->next;
}
};