Code shared from the Rust Playground

playground.rs

fn collatz(num: i64, list: &mut Vec<i64>) {
    let exists: bool = list.contains(&num);
    match exists {
        true => {}
        false => {
            list.push(num);
            let even: bool = num % 2 == 0;
            match even {
                true => collatz(num / 2, list),
                false => collatz(3 * num + 1, list),
            }
        }
    }
}

fn main() {
    for x in -42..=42 {
        let mut list: Vec<i64> = Vec::new();
        collatz(x, &mut list);
        println!("collatz of {}: {:?}", x, &mut list)
    }
}