19. Remove Nth Node From End of List

Given a linked list, remove the nth
node from the end of list and return its head.
For example,
Given linked list: 1->2->3->4->5, and n = 2. After removing the second node from the end, the linked list becomes 1->2->3->5.
Note:Given n will always be valid.Try to do this in one pass.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        if(head == null)
          return null;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode p = dummy;
        ListNode q = dummy;
        
        for(int i=0;i<n;i++)
         {  
             p = p.next;
             if(p==null)
               return null;
         }
        while(p.next!=null)
         {
             p = p.next;
             q = q.next;
         }
         
         
         q.next = q.next.next;
         return dummy.next;
        
    }
}