Equilateral numbers

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

using Z = long long int;

int main() {
    cin.sync_with_stdio(false);
    cin.tie(nullptr);

    Z n;
    cin >> n;

    Z i = 1;
    Z best = 3;
    while(true) {
        Z x = i * (i + 1) / 2;
        if(x > n) {
            break;
        }

        if(x == n) {
            best = 1;
            break;
        } else {
            Z diff = n - x;

            Z A = 0;
            Z B = 2000000;
            while(A != B) {
                Z M = (A + B + 1) / 2;
                if(M * (M + 1) / 2 > diff) {
                    B = M - 1;
                } else {
                    A = M;
                }
            }
            if(diff == A * (A + 1) / 2) {
                best = 2;
            }
        }

        ++i;
    }
    cout << best << "\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