Swap Nodes in Pairs

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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.

这个问题我们先建一个头结点，这样可以统一操作，每一步就那么几个操作，见图。

这样就不用特别处理第一结点，好了看明白这个图说的步骤就代码。

class Solution {public:ListNode *swapPairs(ListNode *head) {if(head == NULL || head->next == NULL)return head;ListNode *pre, *tail;ListNode *new_head = new ListNode(-1);new_head->next = head;pre = new_head;while(pre->next && pre->next->next){tail = pre->next->next;pre->next->next = tail->next;tail->next = pre->next;pre->next = tail;pre = tail->next;}pre = new_head;new_head = new_head->next;delete pre;return new_head;}};

leetcode里有好多链表题，这里是创建链表的函数，想自己测试的在这里下：创建链表函数

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