Swap Nodes in Pairs

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;
        }
};