fn main() {}

pub fn series(digits: &str, len: usize) -> Vec<String> {
    let mut series = vec![];
    if len > digits.len() {
        return series;
    }
    let mut chars = digits.chars();
    for _ in 0..=(digits.len() - len) {
        series.push(chars.clone().take(len).collect());
        chars.next();
    }
    series
}