leetcode 24. Swap Nodes in Pairs

1.题目

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.

2.思路 

画个图就方便理解了。

class Solution {public:    ListNode* swapPairs(ListNode* head) {        ListNode* newhead = new ListNode(-1);        newhead->next = head;        ListNode* pre = newhead;        ListNode* 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 newhead->next;    }};