【杭电acm】1016 Prime Ring Problem

#include <stdio.h>
#define false 0
#define true  1

int is_prime[41];
int is_visit[21];
int sequence[21];

void DFS(int, int, int);

int main() {

    int i, j, k;
    int n;

    is_prime[1] = false;
    is_prime[2] = true;


    for (i=2; i<=40; i++) {
        k = 0;
        for (j=2; j*j<=i; j++) {
            if (i%j == 0) {
                k = 1;
                break;
            }
        }
        if (k) {
            is_prime[i] = false;
        } else {
            is_prime[i] = true;
        }

    }
    
    k = 0;
    
    while (scanf("%d", &n) != EOF) {
        printf("Case %d:\n", ++k);
        
        DFS(1, 1, n);
        
        printf("\n");
    }

    return 0;
}

void DFS(int num, int order, int end) {
    int i;
    
    is_visit[num] = true;
    sequence[order] = num;
    
    if (order == end) {  
        if (is_prime[num+1]) {
            sequence[order] = num;
            for (i=1; i<=end; i++) {
                if (i != end)
                    printf("%d ", sequence[i]);
                else
                    printf("%d\n", sequence[i]);
            }                        
        }    
    } else {
        for (i=1; i<=end; i++) {
            if (is_visit[i] == false && is_prime[i+num]) {
                DFS(i, order+1, end);
                is_visit[i] = false;
            }
        }
    }
}