C :

#include <stdio.h>
main()
{
  int n, m=3, i, s=0;
  scanf("%d", &n);
  for (i=2; i<=n; i++) s=(s+m)%i;
  printf ("%d\n", s+1);
}

C++ :

#include <stdio.h>
#include <stdlib.h>
struct node {
	int num;
	struct node * next;
};
int main() {
	struct node *p, *head, *current;
	int n, i;
	scanf("%d", &n);
	/* 建立循环链表 */
	head = current = NULL;
	for (i = 0;i < n;i++) {
		p = (struct node *)malloc(sizeof(struct node));
		p->num = i + 1;
		if (head == NULL) {
			head = current = p;
		} else {
			current->next = p;
			current = p;
		}
	}
	current->next = head;
	/* 模拟前n-1个人报号退出圈子的过程 */
	current = head;
	for (i = 0;i < n - 1;i++) {
		current = current->next;
		p = current->next;
		current->next = p->next;
		current = current->next;
		free(p);
	}
	printf("%d\n", current->num);
	return 0;
}