HDU 1016 Prime Ring Problem



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

回溯法

#include <math.h>
#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>

using namespace std;

bool isp[50],vis[50];
int n,a[50];
void IsPrime(int x){
    bool flag = true;
    for(int i=2; i<=sqrt(x); i++)
        if(!(x % i)) {flag = false; break;}
    if(flag)    isp[x] = true;
    return;
}

void DFS(int cur){
    if(cur == n+1 && isp[a[n]+a[1]]){
        printf("%d",a[1]);
        for(int i=2; i<=n; i++)
            printf(" %d",a[i]);
        printf("\n");
    }
    else{
        for(int i=2; i<=n; i++){
            if(!vis[i] && isp[i+a[cur-1]]){
                a[cur] = i;
                vis[i] = 1;
                DFS(cur+1);
                vis[i] = 0;
            }
        }
    }
}

int main(){
//    freopen("in.txt", "r", stdin);
    for(int i=1; i<=40; i++)    {isp[i] = false; vis[i] = false;}
    for(int i=2; i<=40; i++)    IsPrime(i);
    int t = 0;
    while(scanf("%d",&n) == 1){
        printf("Case %d:\n",++t);
        a[1] = 1;
        DFS(2);
        printf("\n");
    }
    return 0;
}