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.

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思路：设置一个头结点，每俩个节点倒置一下

/** * 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 *ans=new ListNode(0);          ans->next=head;          ListNode *cur=ans;          while(cur && cur->next && cur->next->next){              ListNode *first=cur->next;        	  ListNode *second=first->next;        	  ListNode *three=second->next;        	  first->next=three;        	  second->next=first;        	  cur->next=second;        	  cur=first;           }          return ans->next;        }};