剑指 Offer II 021. 删除链表的倒数第 n 个结点【链表】

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(0, head);
        ListNode first = head;
        ListNode second = dummy;
        for (int i = 0; i < n; ++i) {
            first = first.next;
        }
        while (first != null) {
            first = first.next;
            second = second.next;
        }
        second.next = second.next.next;
        ListNode ans = dummy.next;
        return ans;//这里如果返回head的话，那么当链表只有一个元素并且删除一个元素的时候返回的不是空
    }
}