LeetCode *19. 删除链表的倒数第 N 个结点（双指针）

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode *first = head;
        ListNode *second = head;
        int m = 0;
        while (first->next != nullptr && m < n) {
            m++;
            first = first->next;
        }
        if (m != n) {
            return head->next;
        }
        while (first->next != nullptr) {
            second = second->next;
            first = first->next;
        }
        second->next = second->next->next;
        return head;
    }
};