题目：

题解：

struct ListNode* removeNthFromEnd(struct ListNode* head, int n) {
    struct ListNode* dummy = malloc(sizeof(struct ListNode));
    dummy->val = 0, dummy->next = head;
    struct ListNode* first = head;
    struct ListNode* second = dummy;
    for (int i = 0; i < n; ++i) {
        first = first->next;
    }
    while (first) {
        first = first->next;
        second = second->next;
    }
    second->next = second->next->next;
    struct ListNode* ans = dummy->next;
    free(dummy);
    return ans;
}