// 再帰的に構築したくない？
// AA AB BB BA
// AA [AC CB BC CC CA] AB BB BA
// AA [AD DD DC CD DB BD DA] AC CB BC CC CA AB BB BA

fn main() {
    const K: u8 = 26;
    let mut ans = vec![(0, 0)];
    for i in (1..K).rev() {
        ans.push((0, i));
        ans.push((i, i));
        for j in (1..i).rev() {
            ans.push((i, j));
            ans.push((j, i));
        }
        ans.push((i, 0));
    }

    assert_eq!(ans.len(), K as usize * K as usize);
    assert!(ans.windows(2).all(|w| w[0].1 == w[1].0));
    assert_eq!(ans.last().unwrap().1, ans[0].0);

    for &(a, b) in &ans {
        let a = (a + b'A') as char;
        let b = (b + b'A') as char;
        println!("{a}{b}");
    }
}