链表删除倒数第N个元素

public ListNode removeNthFromEnd(ListNode head, int n) {
    if(head == null){
        return null;
    }

    if(head.next == null){
        return null;
    }                                    



    ListNode ans = head;
    ListNode cur = head;
    
    for(int i = 0; i < n; i++){
        if(cur.next == null){
            if(i == n-1){
                return head.next;
            }
            return null;
        }
        else{
            cur = cur.next;
        }
    }

    while(cur.next != null){
        ans = ans.next;
        cur = cur.next;
    }

    ans.next = ans.next.next;

    
    return head;
}