19 删除链表的倒数第 N 个结点

https://leetcode.cn/leetbook/read/top-interview-questions-easy/xn2925/

https://leetcode.cn/problems/remove-nth-node-from-end-of-list/description/

cost: 13m32s

/**
 * 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 = nullptr;
        while (first != nullptr) {
            first = first->next;
            if (n > 0) {
                n -= 1;
                continue;
            }
            if (second == nullptr) {
                second = head;
            } else {
                second = second->next;
            }
        }
        if (second == nullptr) {
            return head->next;
        }
        second->next = second->next->next;
        return head;
    }
};

• 时间复杂度：O(n)
• 空间复杂度：O(1)