输入一个链表，输出该链表中倒数第K个结点

public class ListNode {    int val;    ListNode next = null;    ListNode(int val) {        this.val = val;    }

public class Solution {    public ListNode FindKthToTail(ListNode head,int k) {        ListNode p = head;        ListNode pre = head;        int a =k;        int count = 0;              while(p != null){            p = p.next;            count ++;            if(k <1){                pre = pre.next;            }            k--;        }        if(count < a){            return null;        }        return pre;    }}