Swap Nodes in Pairs

PROBLEM :

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.

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SIMPLE c++ IMPLEMENTATION :
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/**
 * 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* one,*two,*three ;
        one=head ;
        two=one->next ;
        three=two->next ;
       
        one->next=NULL ;
        two->next=one ;
       
        if(three!=NULL)
            one->next=swapPairs(three) ;
        return two ;
    }
};

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