二分图不是一个无向图吗，为什么这题建双向边反而会出错，单向边才能AC

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pll;
const int N = 1e4 + 10, M = 2e5 + 10;
int n, m, ans;
int h[N], e[M], ne[M], idx;
int match[N];
bool st[N];
void add(int a, int b) {
	e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}
bool find(int u) {
	for (int i = h[u]; ~i; i = ne[i]) {
		int j = e[i];
		if (!st[j]) {
			st[j] = 1;
			if (!match[j] || find(match[j])) {
				match[j] = u;
				return true;
			}
		}
	}
	return false;
}
int main() {
	cin >> n >> m;
	memset(h, -1, sizeof h);
	for (int i = 1 ; i <= n ; ++i) {
		int s;
		cin >> s;
		for (int j = 1; j <= s; ++j) {
			int x;
			cin >> x;
			add(i, x);
		}
	}
	for (int i = 1 ; i <= n ; ++i) {
		memset(st, 0, sizeof st);
		if (find(i)) ans++;
	}
	cout << ans << endl;
	return 0;
}