[LeetCode] 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.

/**
 * 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) {
    
       if(head== NULL || head->next == NULL)
           return head;

       ListNode *p1 = head;
       head = head->next;
       ListNode *p2 = p1->next;
       ListNode *newh = p2->next;
       p2->next = p1;

       while(newh)
       {
          
          if( newh->next)
          {
             p2 = newh->next;
             p1->next = p2;
             p1 = newh;
             newh = p2->next;
             p2->next = p1;
          }
          else 
          {
             p1->next = newh;
             p1 = newh;
             p2 = NULL;
             newh->next = NULL;
             break;
          }
          
       
       }
      p1->next = NULL;

       return head;
    }
};

ps：不要忘了链表末尾要指向NULL。