fn main() {
    for s in vec![1, 3, 20, 61, 3601, 67823430] {
        println!("{} -> {}", s, duration(s));
    }
}

fn duration(s: u64) -> String {
    vec![s / 60 / 60, (s / 60) % 60, s % 60]
        .into_iter()
        .skip_while(|c| *c == 0)
        .map(|c| format!("{:02}", c))
        .collect::<Vec<String>>()
        .join(":")
}