fn parse_usize(mut s: &str) -> Option<usize> {
    let radix = if s.starts_with("0x") {
        s = &s[2..];
        16
    } else if s.starts_with("0o") {
        s = &s[2..];
        8
    } else if s.starts_with("0b") {
        s = &s[2..];
        2
    } else {
        10
    };

    usize::from_str_radix(s, radix).ok()
}

fn main() {
    let n = parse_usize("42").unwrap();

    println!("{}", n);
}