fn s2n(s: &str) -> u32 {
    s.bytes().fold(0, |acc, b| acc * 26 + (b - b'A'+ 1) as u32)
}
fn n2s(n: u32) -> String {
    fn aux(mut acc: String, n: u32) -> String {
        if n < 1 {acc} else {
            acc.insert(0, (((n - 1) % 26) as u8 + b'A') as char);
            aux(acc, (n - 1) / 26)
        }
    }
    aux("".to_string(), n)
}
fn main() {
    for s in vec!("A", "AA", "ABC") {println!("{} => {}", s, s2n(s))};
    for n in vec!(26, 266379) {println!("{} => {}", n, n2s(n))}
}
 

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