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给你一个链表，删除链表的倒数第

n



个结点，并且返回链表的头结点。

示例 1：

输入：head = [1,2,3,4,5], n = 2
输出：[1,2,3,5]

示例 2：

输入：head = [1], n = 1
输出：[]

示例 3：

输入：head = [1,2], n = 1
输出：[1]

/**
 * 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){
        int pos=length(head, n);
        if(pos==n)
            return head.next;
        return head;
    }

    public int length(ListNode node,int n){
        if(node==null) return 0;
        int pos=length(node.next,n)+1;
        if(pos==n+1)
            node.next=node.next.next;
        return pos;
    }
}

/**
 * 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 pre=head;
        int last=length(head)-n;
        if(last==0) return head.next;
        for(int i=0;i<last-1;i++) pre=pre.next;

        pre.next=pre.next.next;
        return head;
    }

    private int length(ListNode head) {
        int len=0;
        while(head!=null) {
            len++;
            head=head.next;
        }
        return len;
    }
}