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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/**
* 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 * myheader =head;
ListNode * first = head;
ListNode * root = head;
while(first!=NULL)
{
ListNode * second = first->next;
if(second!=NULL)
{
if(first == head)
{
//head == second;
root = second;
first->next = second->next;
second->next = first;
myheader = first;
}
else
{
myheader->next = second;
first -> next = second->next;
second->next = first;
myheader = first;
}
first = first ->next;
}
else
{
break;
}

}
};