Kolmijako

use std::io::{self, BufRead};

// Tarvitaan 000 -> 012
// Ja 012 -> 000

// t == true : 012
// t == false : 000
fn get_arr(t: bool, cnt: usize) -> Vec<usize> {
    match (cnt, t) {
        (5, true) => return vec![1, 3, 1, 3, 2],
        (5, false) => return vec![1, 2, 2, 1, 3],
        (6, true) => return vec![2, 2, 3, 2, 3, 1],
        (6, false) => return vec![3, 2, 1, 1, 2, 3],
        _ => {}
    }
    if cnt < 5 {
        panic!()
    }
    if t {
        let mut arr = get_arr(false, cnt - 3);
        arr.push(1);
        arr.push(2);
        arr.push(3);
        return arr
    } else {
        let mut arr = get_arr(true, cnt - 3);
        arr.push(3);
        arr.push(2);
        arr.push(1);
        return arr
    }
}

fn main() {
    let stdin = io::stdin();
    let mut lines = stdin.lock().lines();
    let n = lines.next().unwrap().unwrap().parse::<usize>().unwrap();
    if n % 3 == 1 {
        println!("IMPOSSIBLE");
        return;
    }
    match n {
        0..=2 => panic!(),
        3 => println!("1 2 3"),
        _ => {
            let res = get_arr(true, n);
            for d in res {
                print!("{} ", d);
            }
            println!();
        }
    }
}

Test 1

Group: 1, 2

Verdict: ACCEPTED

Test 2

Group: 1, 2

Verdict: ACCEPTED

Test 3

Group: 1, 2

Verdict: ACCEPTED

Test 4

Group: 1, 2

Verdict: ACCEPTED

Test 5

Group: 1, 2

Verdict: ACCEPTED

Test 6

Group: 1, 2

Verdict: ACCEPTED

Test 7

Group: 1, 2

Verdict: ACCEPTED

Test 8

Group: 1, 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 2

Verdict: ACCEPTED

Test 12

Group: 2

Verdict: ACCEPTED

Test 13

Group: 2

Verdict: ACCEPTED

Test 14

Group: 2

Verdict: ACCEPTED

Test 15

Group: 2

Verdict: ACCEPTED