How to calculate time complexity for this function

ListNode  FindnthToLast(int  n)  {

    	if (n <1 || headNode == null)
    	  return null;

    	ListNode pntr1 = headNode, pntr2 = headNode;

    	//advance pntr2 by n-1 nodes;    
    	for  (int  i  =  0;  i  <  n  - 1;  ++i)  {  

    	if  (pntr2  ==  null)  {
    	  return null; 
    	  }

    	else //go to the next node
    	   pntr2 = pntr2.getNext();

    	}


    	while(pntr2.getNext() != null)
    	{
    	  pntr1 = pntr1.getNext();
    	  pntr2 = pntr2.getNext();
    	}

    	return pntr1;

    	}