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.
	* public class ListNode {
	* int val;
	* ListNode next;
	* ListNode(int x) {
	* val = x;
	* next = null;
	* }
	* }
	*/
	public class Solution {
	public ListNode swapPairs(ListNode head) {
	// Start typing your Java solution below
	// DO NOT write main() function
	if (head==null) return head;
	ListNode helper=new ListNode(0);
	helper.next=head;
	ListNode n1=helper;
	ListNode n2=head;
	while(n2!=null&& n2.next!=null){
	n1.next=n2.next;
	n2.next=n2.next.next;
	n1.next.next=n2;
	n1=n2;
	n2=n2.next;
	}
	return helper.next;
	}
	}

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