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.

Subscribe to see which companies asked this question

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;        }};