Prime factorization with sieve of Eratosthenes

fact.cpp

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

vector<vector<ll>> sieve(ll n){
    vector<vector<ll>> fact(n + 1);
    for(ll i = 2; i <= n; i++){
        if(fact[i].empty()){
            for(ll j = i; j <= n; j += i){
                fact[j].push_back(i);
            }
        }
    }
    return fact;
}

int main(){
    auto fact = sieve(4);
    for(ll i : fact[4]) cout << i << ' ';
}