fn digit_to_chars(digit: char) -> Vec<char> {
    match digit {
        '2' => vec!['a', 'b', 'c'],
        '3' => vec!['d', 'e', 'f'],
        '4' => vec!['g', 'h', 'i'],
        '5' => vec!['j', 'k', 'l'],
        '6' => vec!['m', 'n', 'o'],
        '7' => vec!['p', 'q', 'r', 's'],
        '8' => vec!['t', 'u', 'v'],
        '9' => vec!['w', 'x', 'y', 'z'],
        _ => panic!(
            "Character out of range: '{}' is not between '2' and '9'.",
            digit
        ),
    }
}

pub fn combination(digits: &str) -> Vec<String> {
    let mut combinations: Vec<String> = vec![String::new()];

    for d in digits.chars() {
        let mut temp = Vec::new();
        for c in combinations {
            let chars = digit_to_chars(d);
            for ch in chars {
                temp.push(format!("{}{}", c, ch));
            }
        }
        combinations = temp;
    }

    if combinations.len() == 1 {
        vec![] as Vec<String>
    } else {
        combinations
    }
}