Kolmijako

#include <bits/stdc++.h>

using namespace std;

#define N 101

int n;
int z[N];

void collect(int s, int v)
{
    int sum = 0;
    for(int i=n;i>0;i--)
    {
        if(z[i]) continue;
        if(sum+i<=s)
        {
            sum += i;
            z[i] = v;
        }
        if(sum==s) break;
    }
}

int main()
{
    cin>>n;
    int x = (n+1)*n/2;

    if(x%3==0)
    {
        x /= 3;
        x--;
        collect(x, 1);
        x++;
        collect(x, 2);
        x++;
        collect(x, 3);

        for(int i=1;i<=n;i++)
        {
            cout<<z[i]<<" ";
        }
    }
    else
    {
        cout<<"IMPOSSIBLE";
    }
}

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