题解 | #删除链表的倒数第n个节点#

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 *	ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @param n int整型 
     * @return ListNode类
     */
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        // write code here
        if (head == nullptr) {
            return nullptr; 
        }
        if (n <= 0 || n > 1000) {
            return nullptr;
        }

        ListNode* preHead = new ListNode(-1);
        preHead->next = head;

        ListNode* first = head;
        ListNode* last = head;
        ListNode* pre = preHead;

        for(int i = 1; i < n; i++) {
            if(last->next == nullptr) {
                return nullptr;
            }
            last = last->next;
        } // last为第n个节点

        
        while (last->next != nullptr) {
            pre = pre->next;
            first = first->next;
            last = last->next;
        } // first为倒数第n个节点

        // 删除first
        pre->next = first->next;
        first->next = nullptr;
        delete(first);

        return preHead->next;
    }
};