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[21];
int n, m, dp[1 << 21];
int main()
{
    int p, x;
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= n; i++)
    {
        scanf("%d", &p);
        for (int j = 0; j < p; j++)
        {
            scanf("%d", &x);
            G[i].push_back(x);
        }
    }
    dp[0] = 1;
    for (int i = 0; i < n; i++)
    {
        for (int j = (1 << m + 1) - 1; j >= 0; j--)
        {
            if (__builtin_popcount(j) != i) continue;
            for (int k = 0; k < G[i + 1].size(); k++)
            {
                int tmp = G[i + 1][k];
                if (!(j >> tmp & 1)) dp[j | 1 << tmp] += dp[j];
            }
        }
    }
    int cnt = 0;
    for (int j = 0; j < (1 << m + 1) - 1; j++) 
        if (__builtin_popcount(j) == n) cnt += dp[j];
    printf("%d
", cnt);
    return 0;
}