Equilateral numbers

#include <bits/stdc++.h>

using namespace std;

#define rep(i, a, b) for (int i = (a); i < (b); i++)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

int main() {
	cin.tie(0)->sync_with_stdio(0);
	cin.exceptions(cin.failbit);
	
	ll n;
	cin >> n;

	vector<ll> sqrs = {1};
	for (ll i = 2; ; i++) {
		if (sqrs.back() + i > n) break;
		sqrs.push_back(sqrs.back()+i);
	}

	unordered_set<ll> x{all(sqrs)};

	if (x.count(n)) {
		cout << 1 << endl;
	} else {
		for (auto y : sqrs) {
			if (x.count(n - y)) {
				cout << 2 << endl;
				return 0;
			}
		}
		cout << 3 << endl;
	}
}

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