leetcode Remove Nth Node From End of List

双指针大法又显神通。

/**
 * 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) {
        		 if(head==null)return null;
		 ListNode ret=new ListNode(-1);
		 ret.next=head;
		 ListNode p1=ret;
		 ListNode p2=ret;
		 int i=0;
		 while(i<n&&p2.next!=null){
			 p2=p2.next;
			 i++;
		 }
		 if(p2==null&&i!=n)return head;
		
		 while(p2.next!=null){
			 p1=p1.next;
			 p2=p2.next;
			 
		 }
		 p1.next=p1.next.next;
		 return ret.next;
       
    }
}