Equilateral numbers

#include <bits/stdc++.h>
using namespace std;
#define endl "\n";
 
#define PRINT_ARR(arr, n) \
    do { \
        for (int i = 0; i < n; i++) { \
            cout << (arr)[i] << " "; \
        } \
        cout << endl; \
    } while (0)
 
typedef long long ll;
 
unordered_set<ll> S;
 
int main() {
    ll n;
    cin >> n;
    for (ll i = 1; i*(i+1)/2 <= n; ++i) {
        S.insert(i*(i+1)/2);
    }
    if (S.count(n)) {
        cout << "1\n";
        return 0;
    }
    for (ll c : S) {
        if (S.count(n - c)) {
            cout << "2\n";
            return 0;
        }
    }
    cout << "3\n";
    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED