P2417 Courses

There are $n$ students and $m$ classrooms. For classroom $i$, there are $k_i$ students who can take a class there, with indices $p_{i,j}$. Each student has at least one classroom they **can** attend, and each student can attend **only** one classroom. Determine whether there exists an assignment such that each classroom has at least one student. If it is possible, output `YES`; otherwise, output `NO`.

The first line contains the number of test cases $T$, where $T \leq 10$. For each test case, the first line contains two integers $m, n$ (note the input order). Then follow $m$ lines: the first number is $k_i$, followed by $k_i$ numbers $p_{i,j}$, indicating that student $p_{i,j}$ can take a class in classroom $i$.

For each test case, output one string `YES` or `NO`, indicating whether there exists a valid assignment for the corresponding testdata.

Constraints: $n \leq 2 \times 10^4$, $m \leq 2 \times 10^4$, $T \leq 10$. Translated by ChatGPT 5