hamming: `Result` should be `usize`
This is effectively a difference of two sizes, so we should return `usize`
/// Return the Hamming distance between the strings,
/// or an error if the lengths are mismatched.
-pub fn hamming_distance(s1: &str, s2: &str) -> Result<u64, ()> {
+pub fn hamming_distance(s1: &str, s2: &str) -> Result<usize, ()> {
unimplemented!("What is the Hamming Distance between {:?} and {:?}",
s1, s2);
}
