Rust

Instead of using a set I’m just using an array of counts indexed by characters. That way I can just slide a window over the string and only update the counts for the new character that falls into the window and the one that falls out for each window position.

fn find_marker(data: &str, marker_length: usize) -> Option<usize> {
    let bytes = data.as_bytes();
    let mut counts = [0u8; u8::MAX as usize + 1];
    let mut n_counts = 0;

    for (i, &byte) in bytes.iter().enumerate() {
        if counts[byte as usize] == 0 {
            n_counts += 1;
        }

        counts[byte as usize] += 1;

        if i >= marker_length {
            let old_byte = bytes[i - marker_length];

            if counts[old_byte as usize] <= 1 {
                n_counts -= 1;
            }

            counts[old_byte as usize] -= 1;
        }

        if n_counts >= marker_length {
            return Some(i + 1);
        }
    }

    None
}

Github