fn caesar_cipher(s: &str, k: i32) -> String {
    //Time complexity: O(n)
    //Space complexity (ignoring input): O(n)
    let mut new_string = String::with_capacity(s.len());
    let k = k as u8;
    for letter in s.chars() {
        if letter.is_alphabetic() {
            let sum_a = if letter.is_uppercase() { b'A' } else { b'a' };
            new_string.push(((letter as u8 - sum_a + k) % (b'z' - b'a' + 1) + sum_a) as char);
        } else {
            new_string.push(letter);
        }
    }
    new_string
}