24. Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

• Your algorithm should use only constant extra space.
• You maynotmodify the values in the list's nodes, only nodes itself may 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) {
    
      
        
        ListNode *head_1=(ListNode*)malloc(sizeof(ListNode));
        head_1->next=head;
        
         
          ListNode *p=head_1;
          ListNode *r=p->next;
        if(r==NULL) return  head_1->next;
          ListNode *q=r->next;
     if(q==NULL) return  head_1->next;
        while(p)
        {
            
          
            p->next=q;
            
           r->next=q->next;
           
           q->next=r;
            p=r->next;
          
         p=r; 
            if(p->next==NULL) break;
         r=p->next;
         q=r->next; 
             if(q==NULL) break;
        }
        
         
           
        
        return head_1->next;
        
        
    }
};