一、不使用虚拟头结点

import java.util.*;

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

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param pHead ListNode类 
     * @param k int整型 
     * @return ListNode类
     */
    public ListNode FindKthToTail (ListNode pHead, int k) {
        // write code here
        ListNode fast = pHead, slow = pHead;
        for(int i = 0; i < k; i ++){
            if(fast == null) return fast;
            fast = fast.next;
        }
        while(fast != null){
            fast = fast.next;
            slow = slow.next;
        }
        return slow;
    }
}

二、使用虚拟头结点

import java.util.*;
 
/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */
 
public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param pHead ListNode类
     * @param k int整型
     * @return ListNode类
     */
    public ListNode FindKthToTail (ListNode pHead, int k) {
        // write code here
        ListNode dummy = new ListNode(-1);
        dummy.next = pHead;
        ListNode fast = dummy;
        ListNode slow = dummy;
 
        for(int i = 0; i <= k; i ++){
            if(fast == null) return fast;
            if(fast != null) fast = fast.next;
        }
 
        while(fast != null){
            fast = fast.next;
            slow = slow.next;
        }
 
        return slow.next;
    }
}