19.cpp

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) {
        if (head->next == nullptr) return nullptr;

        ListNode* dummy = new ListNode(0, head);

        ListNode* slow = dummy;
        ListNode* fast = dummy;

        for (int i=0; i<=n; i++) {
            fast = fast->next;
        }

        while (fast != nullptr) {
            fast = fast->next;
            slow = slow->next;
        }

        slow->next = slow->next->next;

        return dummy->next;
    }
};