pub fn is_isomorphic(a: String, b: String) -> bool {
  let mut map_a = std::collections::HashMap::new();
  let mut map_b = std::collections::HashMap::new();

  for (a, b) in a.chars().zip(b.chars()) {
    let from_map_a = *map_a.entry(a).or_insert(b);
    let from_map_b = *map_b.entry(b).or_insert(a);

    if from_map_b != a || from_map_a != b {
      return false;
    }
  }

  true
}