poj2441 Arrange the Bulls

思路：

状态压缩dp。需要一点优化，否则容易超时。

实现：

 #include <cstdio>
 #include <vector>
 #include <cstring>
 #include <iostream>
 using namespace std;
 vector<int> G[];
 int n, m, dp[ << ];
 int main()
 {
     int p, x;
     scanf("%d%d", &n, &m);
     for (int i = ; i <= n; i++)
     {
         scanf("%d", &p);
         for (int j = ; j < p; j++)
         {
             scanf("%d", &x);
             G[i].push_back(x);
         }
     }
     dp[] = ;
     for (int i = ; i < n; i++)
     {
         for (int j = ( << m + ) - ; j >= ; j--)
         {
             if (__builtin_popcount(j) != i) continue;
             for (int k = ; k < G[i + ].size(); k++)
             {
                 int tmp = G[i + ][k];
                 if (!(j >> tmp & )) dp[j |  << tmp] += dp[j];
             }
         }
     }
     int cnt = ;
     for (int j = ; j < ( << m + ) - ; j++)
         if (__builtin_popcount(j) == n) cnt += dp[j];
     printf("%d\n", cnt);
     return ;
 }