fn main() {
    let mut f = std::collections::HashMap::new();
    f.insert(String::from("B"), vec![String::from("A"), String::from("B"), String::from("C"), String::from("D")]);
    f.insert(String::from("C"), vec![String::from("E"), String::from("F"), String::from("G"), String::from("H")]);

    let mut h = f.values().flatten().cloned().collect::<Vec<String>>();
    h.sort();
    println!("{:?}", h);
}