From cassidoo's October 18, 2023 newsletter

is_isomorphic.rs

fn is_isomorphic(s1: &str, s2: &str) -> bool {
    let mask1 = str_mask(s1);
    let mask2 = str_mask(s2);

    mask1 == mask2
}

fn str_mask(s: &str) -> Vec<usize> {
    s.chars()
        .fold((None, Vec::<usize>::new()), |(last_c, mut acc), curr_c| {
            if last_c == Some(curr_c) {
                let last = acc.pop().map(|e| e + 1).unwrap_or(1);
                acc.push(last);
            } else {
                acc.push(1);
            }

            (Some(curr_c), acc)
        })
        .1
}

fn main() {
    assert!(is_isomorphic("abb", "cdd"));
    assert!(!is_isomorphic("cassidy", "1234567"));
    assert!(is_isomorphic("cass", "1233"));
}