代码的鲁棒性：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) {
        if (head == null || k <= 0)
            return null;
        ListNode pre = head;
        ListNode last = head;
        for (int i = 1; i < k; i++) {
            if (pre.next != null) {
                pre = pre.next;
            } else {
                return null;
            }
        }
        while (pre.next != null) {
            pre = pre.next;
            last = last.next;
        }
        return last;
    }
}