19.Remove Nth Node From End of List（移除单链表中倒数第N个结点）

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.

public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode res = head;
       
       int len = 0;
       ListNode ahead = head;
       while(ahead!=null){
           len++;
        ahead = ahead.next ;
       }
       
       if(n>len) return null;
       if(len==n){
           return head.next;
       } 
       
       for(int i = 1;i<len-n;i++){
        head = head.next;
       }
       


       head.next = head.next.next;
       return res;
    }
}