LeetCode-Swap Nodes in Pairs

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:

class Solution{
public:
      void swap(ListNode *node1,ListNode *node2){
            node1->next=node2->next;
            node2->next=node1;
      }
      
      ListNode* swapPairs(ListNode *head){
          if(head==NULL || head->next==NULL)  return head;
          ListNode *ppre=NULL,*pre=NULL;
          ListNode *tmp=NULL;
          ListNode *p=head;
          
          while(p){
               pre=p;
               p=p->next;
               if(p->next){
                      tmp=p->next;
               }
               
               if(pre==head) head=p;
               if(ppre) ppre->next=p;
                   
                   ppre=pre;
                   p=tmp;
          }
          return head; 
      }
};