[leetcode] Swap Nodes in Pairs

From : https://leetcode.com/problems/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 || !head->next) return head;
ListNode *pre=head, *post=head->next, *p=new ListNode(0);
head = p;
while(pre && post) {
pre->next = post->next;
post->next = pre;
p->next = post;
p = pre;
pre = pre->next;
if(pre) post = pre->next;
}
p = head;
head = p->next;
delete p;
return head;
}
};