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]

思路

        双指针，快指针判断范围，慢指针定位；哈希；递归。

代码

        递归

class Solution {
public:
    int step = 0;
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        if(head)
            head->next = removeNthFromEnd(head->next, n);
        else
            return nullptr;
        ++step;
        if(step == n)
            return head->next;
        else
            return head;
    }
};

        双指针

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        unordered_map<int, ListNode*> hashmap; 
        int step = 0;
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast!=nullptr)
        {
            fast = fast->next;
            ++step;
        }
        fast = head;
        if((step-=n))
        {
            for(int i=0; i<step; ++i)
            {
                if(i == step-1)
                    slow = fast;
                fast = fast->next;
            }
            slow->next = fast->next;
        }
        else
            head = head->next;
        return head;
    }
};

        哈希

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        unordered_map<int, ListNode*> hashmap; 
        int cnt = 0;
        ListNode* node = head;
        while(node != nullptr)
        {
            ++cnt;
            hashmap[cnt] = node;
            node = node->next;
        }
        if(n!=cnt)
            hashmap[cnt-n]->next =hashmap[cnt-n+1]->next;
        else
            head = head->next;
        return head;
    }
};