solve.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
struct ListNode {
	int val;
	struct ListNode *next;
};
struct ListNode *removeNthFromEnd(struct ListNode *head, int n) {
	if (head == NULL)
		return NULL;
	struct ListNode *p = head, *q = head;
	int i = 0;
	for (i = 0; i < n && p; ++i) p = p -> next;
	struct ListNode *t = NULL;
	if (i < n)
		return head;
	while (p) {
		p = p -> next;
		t = q;
		q = q -> next;
	}
	if (t != NULL) {
		t -> next = q -> next;
		q -> next = NULL;
		free(q);
		q = NULL;
		return head;
	} else {
		head = head -> next;
		free(q);
		q = NULL;
		return head;
	}
}
void init(struct ListNode **head, int a[], int n)
{
	struct ListNode *p = malloc(sizeof(*p));
	p -> val = a[0];
	*head = p;
	for (int i = 1; i < n; ++i) {
		struct ListNode *t = malloc(sizeof(*t));
		t->val = a[i];
		p -> next = t;
		p = t;
	}
}
void print(struct ListNode *head)
{
	struct ListNode *p = head;
	while (p) {
		printf("%d ", p->val);
		p = p->next;
	}
	printf("\n");
}
struct ListNode *rm(struct ListNode *head, int n)
{
	return removeNthFromEnd(head, n);
}
int main(int argc, char **argv)
{
	struct ListNode *head = NULL;
	int a[] = {1,2,3,4,5};
	init(&head, a, 5);
	print(head);
	head = rm(head, 2);
	print(head);
	head = rm(head, 4);
	print(head);
	head = rm(head, 100);
	print(head);
	return 0;
}