Rust HashSet, BTreeMap and VecDeque

use std::collections::{BTreeMap, HashSet, VecDeque};

fn main() {
    let mut seen = HashSet::new();
    for n in [3, 1, 3, 2, 1] {
        if !seen.insert(n) {
            println!("duplicate {}", n);
        }
    }
    println!("{}", seen.len());

    let mut scores = BTreeMap::new();
    scores.insert("sam", 22);
    scores.insert("ada", 36);
    scores.insert("lee", 41);
    for (name, age) in &scores {
        println!("{} {}", name, age);
    }

    let mut queue = VecDeque::new();
    queue.push_back("first");
    queue.push_back("second");
    queue.push_front("zeroth");
    println!("{:?}", queue.pop_front());
    println!("{:?}", queue);
}

duplicate 3
duplicate 1
3
ada 36
lee 41
sam 22
Some("zeroth")
["first", "second"]