HDU1016 Prime Ring Problem

PS: 很基础的一道搜索题目，也不需要特殊的剪枝技术，数据还是挺弱的。

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
bool is_prime(int x);
void getPrime(); // get Prime.
const int maxn = 25;
int prime[50]; // save prime. hash。
int res[maxn]; // save result.
bool vis[maxn]; // mark if visited.
int n;
int Case;
//Accepted 1016 250MS 300K 1392 B G++ Achiberx
void dfs(int parent, int lev) {
if(lev == n+1 && prime[1+res
]==1+res
&& n>1) {
printf("%d", res[1]);
for(int j = 2; j <= n && n>=2; j++) {
printf(" %d", res[j]);
}
printf("\n");
return;
}
int i;
for(i = 2; i <= n; i++) {
int v = i+parent;
if(!vis[i] && prime[v]==v) {
res[lev] = i;
vis[i] = true;
dfs(i, lev+1);
vis[i] = false;
}
}
}

int main()
{
getPrime();
Case = 1;
while(scanf("%d", &n)!= EOF) {
memset(res, 0, sizeof(res));
memset(vis, false, sizeof(vis));
res[1] = 1;
vis[1] = true;
printf("Case %d:\n", Case++);
dfs(1, 2);// parent, nextIndex.
printf("\n");
}
return 0;
}

bool is_prime(int x) {
for(int i = 2; i <= x/2; i++) {
if(x%i==0 && x!=i) return false;
continue;
}
return true;
}
void getPrime() {
for(int i = 2; i <= 45; i++) {
bool ok = is_prime(i);
if(ok){
prime[i] = i;
}
else prime[i] = -1;
}
}