leetcode 24. 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) {
ListNode* dummy=new ListNode(0);
dummy->next=head;
ListNode* pre=dummy,* cur=head;
while(cur&&cur->next){
pre->next=cur->next;
cur->next=cur->next->next;
pre->next->next=cur;
pre=cur;
cur=cur->next;
}
return dummy->next;

}
};