/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode getKthFromEnd(ListNode head, int k) {
        ListNode fast, slow; // 快慢指针
        fast = slow = head;
        // 先让快指针走k步
        while (k-- > 0) {
            fast = fast.next;
        }
        // 然后快慢指针再同时出发
        // 这样做的目的是，快指针永远在前，只要它走到链表末尾，那么它身后落后k个身位的慢指针的位置就是倒数第k个节点
        while (fast != null) {
            fast = fast.next;
            slow = slow.next;
        }
        return slow;
    }
}