hdu1016 Prime Ring Problem

dfs，用全局数组和变量保存并更新当前状态。

答案可以直接在搜索结束时打印，n为奇数时方案数为0。

acm.hdu.edu.cn/showproblem.php?pid=1016

#include <cstdio>
#include <cstring>

using namespace std;

const int maxn = 20;
int n;
int ans[maxn], k;
bool isprime[50];
bool vis[20];

void init(){
    memset(isprime, 0, sizeof isprime);
    isprime[2] = isprime[3] = 1;
    for(int i = 5; i <= 50; i += 2){
        bool ok = 1;
        for(int j = 3; j * j <= i; j += 2){
            if(i % j == 0){
                ok = 0;
                break;
            }
        }
        isprime[i] = ok;
    }
    memset(vis, 0, sizeof vis);
}

void dfs(int u){
    //finished num is u
    if(u == n && isprime[1 + ans[n - 1]]){
        printf("%d", ans[0]);
        for(int i = 1; i < k; i++) printf(" %d", ans[i]);
        printf("
");
    }
    for(int i = 1; i <= n; i++){
        if(!vis[i] && isprime[i + ans[u - 1]]){
            vis[i] = 1;
            ans[k++] = i;
            dfs(u + 1);
            vis[i] = 0;
            --k;
        }
    }
}

void solve(){
    init();
    k = 0;
    ans[k++] = 1;
    vis[1] = 1;
    dfs(1);
}

int main(){
    int kase = 0;
    while(~scanf("%d", &n)){
        printf("Case %d:
", ++kase);
        solve();
        printf("
");
    }
    return 0;
}