19.cc

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

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