题解 | #删除链表的倒数第n个节点#

虽然题目对于边界条件有限定，但是这里还是考虑各种边界异常情况

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
      if (n <= 0 || head == nullptr) {
        return head;
      }
      
      // 头节点对于边界的处理很重要
      ListNode *head_node = new ListNode(-1);
      head_node->next = head;
      
      ListNode *fast = head_node, *slow = head_node;
      
      for (int i = 0; i < n && fast; i++) {  // 先走 n 步
        fast = fast->next;
      }
      
      if (fast == nullptr) {  // n取值异常
        return head;
      }
      
      while (fast->next) {  // 走到倒数第一个节点
        fast = fast->next;
        slow = slow->next;  // 指向倒数第 n+1 个节点
      }
      
      ListNode *tmp = slow->next;
      slow->next = slow->next->next;
      delete(tmp);
      tmp = nullptr;
      
      head = head_node->next;
      delete(head_node);
      head_node = nullptr;
      
      return head;
    }
};