请问各位dl本蒟蒻的代码为什么只过了2个点？

read()是快读...

谢谢

bool dfs(ll num) {

    while (!A[num].empty()) {
        ll xia = A[num].back();
        A[num].pop_back();
        if (B[xia] == 0 || dfs(B[xia]))return B[xia] = num;
    }
    return false;
}

int main() {
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    n = read() << 1 | 1; A.resize(n), B.resize(n, 0);

    for (i = 1; i != n; ++i)
        j = read() << 1, k = read() << 1,
        A[i].push_back(j), A[i].push_back(j - 1),
        A[i].push_back(k), A[i].push_back(k - 1);
    

    for (i = 1; i != n; ++i)if (dfs(i))++m;

    write(m, '\n');
}

＞﹏＜