#1016：Prime Ring Problem（经典DFS）

原题链接

题意：很容易理解，就是让你输出满足相邻的相加是素数的序列（注意不要重复）

思路就是深搜思想把每种情况遍历一次

代码实现：

#include<iostream>
#include<cstring>
using namespace std;
//素数打表标记每个素数，因为n最大是20，所以只要打到40
bool prime[40] = { 0,1,1,1,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,0,0,0,0,1,0,1,0,0,0,0,0,1,0,0 };
int f[21], n;
bool book[21];
void dfs(int num) {
    if (n == num && prime[f[num - 1] + f[0]]) {
        for (int i = 0; i < num - 1; ++i)
            printf("%d ", f[i]);
        cout << f[num - 1] << endl;
    }
    else {
        for (int i = 2; i <= n; ++i) {
            if (!book[i] && prime[i + f[num - 1]]) {
                book[i] = true;
                f[num++] = i;
                dfs(num);
                book[i] = 0;
                --num;
            }
        }
    }
}
int main() {
    int num = 0;
    while (scanf("%d", &n) != EOF)
    {
        num++;
        printf("Case %d:
", num);
        memset(book, 0, sizeof(book));
        f[0] = 1;
        dfs(1);
        printf("
");
    }

    return 0;
}