public ListNode FindKthToTail (ListNode pHead, int k) {
        // write code here
        if (pHead == null || k < 1) {
            return null;
        }
        // 快指针
        ListNode fast = pHead;
        // 找列表中的第k个节点
        for (int i = 1; i < k; i++) {
            fast = fast.next;
            // 列表长度小于k
            if (fast == null) {
                return null;
            } 
        }
        // 慢指针
        ListNode slow = pHead;
        while (fast.next != null) {
            fast = fast.next;
            slow = slow.next;
        }
        return slow;
    }