找出链表中倒数第K个结点

思路：两个指针，也是快指针和慢指针，先让快指针走k -1步，这时慢指针开始和快指针一起走到尾部。慢指针停止的点就是倒数第k个节点。

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;
    }