Kolmijako

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

fn main() {
    let mut lines = io::BufReader::new(io::stdin()).lines();

    let n = lines.next().unwrap().unwrap().parse::<usize>().unwrap();

    if n % 3 == 1 {
        println!("IMPOSSIBLE")
    }

    let mut gs = Vec::new();
    for k in 1..=n {
        match (k % 6 + 6 - n % 6)%6 {
            0 | 1 => 
                gs.push(2),
            3 | 4 => gs.push(1),
            2 | 5 => gs.push(3),
             _ => ()
        }
    }

    match n % 6 {
        0 => {
            gs[1] = 1;
            gs[2] = 3;
        },
        2 => { 
            gs[0] = 2;
            gs[1] = 3;
        },
        3 => {
            gs[0] = 1;
            gs[1] = 2;
            gs[2] = 3;
        }
        5 => {
            gs[3] = 1;
            gs[1] = 2;
            gs[2] = 2;
            gs[0] = 3;
            gs[4] = 3;
        }
        _ => ()
    }

    for k in gs {
        print!("{} ", k);
    }
    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