Split in Three

#include <bits/stdc++.h>

using namespace std;

#define int long long
#define ff first
#define ss second
#define pb push_back

const int N = 2e5 + 5, inf = 1e18 + 100;

int u[105];

signed main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n;
    cin >> n;

    int s = n * (n + 1) / 2;
    if (s % 3) return cout << "IMPOSSIBLE", 0;

    int x = s / 3 - 1;
    if (x & 1) u[x / 2] = u[x / 2 + 1] = 1;
    else u[x / 2 - 1] = u[x / 2 + 1] = 1;

    for (int i = 1; i <= n; ++i)
    {
        for (int j = 1; j <= n; ++j)
        {
            if (i == j || u[i] || u[j]) continue;
            if (i + j == x + 1)
            {
                u[i] = u[j] = 2;
                break;
            }
            if (i == x + 1) {u[i] = 2; break;}
            if (j == x + 1) {u[j] = 2; break;}
        }
    }

    for (int i = 1; i <= n; ++i)
        cout << (!u[i] ? 3 : u[i]) << ' ';
    return 0;
}

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: WRONG ANSWER

Test 8

Group: 1, 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: WRONG ANSWER

Test 10

Group: 2

Verdict: WRONG ANSWER

Test 11

Group: 2

Verdict: WRONG ANSWER

Test 12

Group: 2

Verdict: ACCEPTED

Test 13

Group: 2

Verdict: WRONG ANSWER

Test 14

Group: 2

Verdict: WRONG ANSWER

Test 15

Group: 2

Verdict: ACCEPTED