Split in Three

#include <bits/stdc++.h>

using namespace std;

#define f first
#define s second

typedef long long ll;
typedef pair<int,int> pi;

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0);
    int n; cin >> n;
    if(n%3 == 1){
        cout << "IMPOSSIBLE\n";
        return 0;
    }
    //find goal
    int goal = (n*(n+1))/6;
    //create set
    vector<int> nums(n+1);
    //find number of moves
    int mv1 = (goal-1)/(n+1); int want1 = (goal-1)%(n+1);
    int mv2 = (goal)/(n+1); int want2 = (goal)%(n+1);
    nums[want1] = 1; nums[want2] = 2;
    int ptr = 1;
    for(int i = 0; i < mv1; ptr++){
        if(nums[ptr] == 0 && nums[n+1-ptr] == 0){
            nums[ptr] = 1; nums[n+1-ptr] = 1; i++;
        }
    }
    for(int i = 0; i < mv2; ptr++){
        if(nums[ptr] == 0 && nums[n+1-ptr] == 0){
            nums[ptr] = 2; nums[n+1-ptr] = 2; i++;
        }
    }
    for(int i = 1; i <= n; i++){
        if(nums[i] == 0) cout << 3 << " ";
        else cout << nums[i] << " ";
    }
    cout << "\n";
    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: 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