[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;    }};