思路：兩個指針，也是快指針和慢指針，先讓快指針走k -1步，這時慢指針開始和快指針一起走到尾部。慢指針停止的點就是倒數第k個節點。

public static ListNode findCountDownNode(ListNode head,int k){
        if(head == null || k <= 0){
            return null;
        }
        ListNode fast = head;   
        for(int i = 0;i < k-1; i++){
            if(fast.next != null
 
) {
                fast = fast.next;
            } else {
                return null;
            }
        }
        ListNode slow = head;
        while(fast.next != null){
            fast = fast.next;
            slow = slow.next;
        }
        return slow;
    }

找出鏈表中倒數第K個結點