奋战杭电ACM（DAY11）1016

DFS加回溯

具体见注释

Prime Ring Problem

#include <iostream>
using namespace std;

int n,circle[20],p[20];
bool visited[20];
int prime[]={1,3,5,7,11,13,17,19,23,29,31,37};//建立素数表，避免每次判断，减少时耗

void print(int x)
{
	for(int i=1; i<x; i++)
		cout << circle[i] << " ";
	cout << circle[x] << endl;//最后一个数后面没有空格，PE一次……
}

bool ifprime ( int y)
{
	for(int i=0; i<12; i++)
	{
		if(y==prime[i])
			return true;
	}
	return false;
}


void dfs(int t)
{
	if(t>n) {print(n);return;}
	for(int i=2; i<=n; i++)
	{
		if(visited[i]==true) continue;
		visited[i]=true;
		circle[t]=p[i];
		if(t==n)
		{
			if(ifprime(circle[t]+circle[t-1]) && ifprime(circle[t]+1))
				dfs(t+1);}
		else
		{if(ifprime(circle[t]+circle[t-1])) dfs(t+1);}
		visited[i]=false;
	}
}


int main()
{
	int num =0;
	while(cin >> n)
	{	
		num +=1;
		memset(circle,0,sizeof(circle));
		memset(visited,false,sizeof(visited));
		for(int i=0; i<20; i++)
			p[i]=i;
		circle[1]=1;
		visited[1]=true;
		cout << "Case " << num << ":" << endl;
		dfs(2);
		cout << endl;
	}
	return 0;
}