URAL 1495 One-two, One-two 2 (BFS)

#include <stdio.h>

int N;
typedef struct Node{
	char num[32];
	int mod;
}Node;
Node queue[1000000];
int visited[1000001];
char *str = "012";
int head, tail;

int strLen(char *str){
	char *p = str;
	while (*p != '\0')
		p++;
	return p - str;
}

void strCpy(char *to, char *from){
	while (*from != '\0'){
		*to = *from;
		from++;
		to++;
	}
	*to = '\0';
}

int main(){

	scanf("%d", &N);
	head = tail = 0;
	Node nodePushed;
	nodePushed.num[0] = '\0';
	nodePushed.mod = 0;
	queue[tail++] = nodePushed;
	while (head < tail){
		Node nodePoped = queue[head++];
		int len = strLen(nodePoped.num);
		if (len > 30)
			break;
		int i;
		for (i = 1; i <= 2; i++){
			int mod;
			mod = (nodePoped.mod * 10 + i) % N;
			if (visited[mod])
				continue;
			//為什麼這樣做标記可以剪枝？
			visited[mod] = 1;
			char num[32];
			strCpy(num, nodePoped.num);
			num[len] = str[i];
			num[len + 1] = '\0';
			if (mod == 0){
				printf("%s\n", num);
				return 0;
			}
			Node nodePushed;
			strCpy(nodePushed.num, num);
			nodePushed.mod = mod;
			queue[tail++] = nodePushed;
			if (head > tail)
				printf("The queue is overflow\n");
		}
	}
	printf("Impossible\n");
	return 0;
}