转换成正序

public class Solution {
    public ListNode FindKthToTail(ListNode head,int k) {
        ListNode node = head;
        // 算出k - length
        while(node != null) {
            k--;
            node = node.next;
        }
        // 算出k对于正序的次序（length + 1 - k）
        k = -k + 1;
        if(k <= 0) {
            return null;
        }
        // 从1开始数
        node = head;
        while(k-- > 1) {
            node = node.next;
        }
        return node;
    }
}

快慢指针

• 快指针先走k步，然后快慢指针同速向前，等快指针到达尾部，慢指针就是倒数第k个

public class Solution {
    public ListNode FindKthToTail(ListNode head,int k) {
        ListNode fast = head, slow = head;
        // 快指针先行k步
        while(k > 0 && fast != null) {
            fast = fast.next;
            k--;
        }
        // k还没走完就说明过长
        if(k > 0) {
            return null;
        }
        // 快慢指针同行
        while(fast != null) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }
}