hiho1087狀態壓縮dp

//記憶話搜索方法

//此題狀態壓縮中套用記憶化搜索的方法

//並用樹狀數組的方法遍歷子集
# include<iostream>
# include<cstdio>
# include<vector>
# include<cstring>
# include<algorithm>
using namespace std;
int n, m, dp[12][1 << 12], st[12], p[1 << 12];
int DP(int u, int s)
{
	if (dp[u][s])
		return dp[u][s];
	if (!s)
		return dp[u][s] = st[u] & 1;
	int rest = s&st[u];
	while (rest) {
		int tp = rest&(-rest);
		dp[u][s] += DP(p[tp], s - tp);
		rest -= tp;
	}
	return dp[u][s];
}
int main()
{
	int a, b;
	while (~scanf("%d%d", &n, &m))
	{
		memset(st, 0, sizeof(st));
		while (m--)
		{
			scanf("%d%d", &a, &b);
			st[a - 1] |= (1 << (b - 1));
		}
		for (int i = 0; i<n; ++i)
			p[1 << i] = i;
		memset(dp, 0, sizeof(dp));
		printf("%d\n", DP(0, (1 << n) - 2));
	}
	return 0;
}