剑指offer四：链表中倒数第k个结点

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;
    }
}