Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode findKthToTail(ListNode pListHead, int k) {
        if(pListHead == null || k < 0 ){return null;}
        ListNode first = pListHead;
        ListNode second = pListHead;
        // 先让first走k个长度
        for(int i = 0; i< k-1 ; i++){
            if(first.next == null){
                return null;
            }
            first = first.next;        
        }
        // 然后first和second都开始走，当first.next==null时结束
        while(first.next != null){
            first = first.next;
            second = second.next;
        }
        return second;
    }
}