use std::iter::once;

fn form_groups(students: &[Vec<usize>]) -> Vec<Vec<usize>> {
    let mut groups: Vec<Vec<usize>> = Vec::new();

    for (student, friends) in students.iter().enumerate() {
        let group = groups.iter_mut().find(|g| g.contains(&student));

        match group {
            None => groups.push(once(student).chain(friends.iter().copied()).collect()),
            Some(g) => {
                g.extend(friends.iter().copied());
            }
        }
    }

    for group in &mut groups {
        group.sort_unstable();
        group.dedup();
    }

    groups
}

fn main() {
    let students = vec![
        vec![1, 2],
        vec![0, 5],
        vec![0],
        vec![6],
        vec![],
        vec![5],
        vec![6],
    ];

    let groups = form_groups(&students);

    println!(
        "the number of friend groups in the class is {};\n{:?}",
        groups.len(),
        groups
    );
}