洛谷1894 [USACO4.2]完美的牛栏The Perfect Stall

原题链接

二分图最大匹配板子。
每个奶牛向它愿意去的牛棚连边，跑二分图最大匹配即可。
这里我用的是匈牙利算法。

#include<cstdio>
#include<cstring>
using namespace std;
const int N = 410;
const int M = N * N;
int fi[N], ne[M], di[M], mtc[N], l;
bool v[N];
inline int re()
{
	int x = 0;
	char c = getchar();
	bool p = 0;
	for (; c < '0' || c > '9'; c = getchar())
		p |= c == '-';
	for (; c >= '0' && c <= '9'; c = getchar())
		x = x * 10 + c - '0';
	return p ? -x : x;
}
inline void add(int x, int y)
{
	di[++l] = y;
	ne[l] = fi[x];
	fi[x] = l;
}
bool dfs(int x)
{
	int i, y;
	for (i = fi[x]; i; i = ne[i])
		if (!v[y = di[i]])
		{
			v[y] = 1;
			if (!mtc[y] || dfs(mtc[y]))
			{
				mtc[y] = x;
				return true;
			}
		}
	return false;
}
int main()
{
	int i, j, n, m, x, s = 0;
	n = re();
	re();
	for (i = 1; i <= n; i++)
	{
		m = re();
		for (j = 1; j <= m; j++)
		{
			x = re();
			add(i, x + n);
		}
	}
	for (i = 1; i <= n; i++)
	{
		memset(v, 0, sizeof(v));
		if (dfs(i))
			s++;
	}
	printf("%d", s);
	return 0;
}