Equilateral numbers

#include <bits/stdc++.h>
using namespace std;

#define int long long
#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;


vi num;
bool Exist(int x)
{
    auto it = lower_bound(all(num), x);
    if(it == num.end())
        return false;
    return (*it) == x;
}

signed main() {
#ifdef LOCAL
freopen(".inp", "r", stdin);
freopen(".out", "w", stdout);
#endif

    int n, now = 0;
    cin >> n;
    
    for (int i = 1; now <= n; i++) {
        now += i;
        num.push_back(now);
    }

    if(Exist(n)) {
        cout << 1;
        return 0;
    }


    for(int x: num)
    {
        int t = n - x;
        if(Exist(t)) {
            cout << 2;
            return 0;
        }

    }

    cout << 3;
    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