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.

 

代码:

1 class Solution{ 2 public: 3       void swap(ListNode *node1,ListNode *node2){ 4             node1->next=node2->next; 5             node2->next=node1; 6       } 7        8       ListNode* swapPairs(ListNode *head){ 9           if(head==NULL || head->next==NULL)  return head;10           ListNode *ppre=NULL,*pre=NULL;11           ListNode *tmp=NULL;12           ListNode *p=head;13           14           while(p){15                pre=p;16                p=p->next;17                if(p->next){18                       tmp=p->next;19                }20                21                if(pre==head) head=p;22                if(ppre) ppre->next=p;23                    24                    ppre=pre;25                    p=tmp;26           }27           return head; 28       }29 };