Remove Nth Node From End of List

fastrunner and slowrunner trick.(be care of head node)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(head == null)
            return null;
        if(n == 1)
        {
            ListNode runner = head;
            if(runner.next == null)
                return null;
            while(runner.next.next != null)
                runner = runner.next;
            runner.next = null;
            return head;
        }
        
        ListNode fastrunner = head;
        ListNode slowrunner = head;
        while(n > 0)
        {
            fastrunner = fastrunner.next;
            n--;
            if(fastrunner == null)
            {
                return head.next;
            }
        }
        while(fastrunner.next != null)
        {
            slowrunner = slowrunner.next;
            fastrunner = fastrunner.next;
        }
        slowrunner.next = slowrunner.next.next;
        return head;
    }
}