Project Euler #12 - Highly divisible triangular number

The sequence of triangle numbers is generated by adding the natural numbers. So the 7^th triangle number would be 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28. The first ten terms would be:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ...

Let us list the factors of the first seven triangle numbers:

 1: 1
 3: 1,3
 6: 1,2,3,6
10: 1,2,5,10
15: 1,3,5,15
21: 1,3,7,21
28: 1,2,4,7,14,28

We can see that 28 is the first triangle number to have over five divisors.

What is the value of the first triangle number to have over five hundred divisors?

출처: https://projecteuler.net/

1부터 n까지의 자연수를 차례로 더하여 구해진 값을 triangle number 이라할 때 500개 이상의 약수를 갖는 가장 작은 triangle number 을 구하는 문제입니다.

루트값의 divisors 갯수를 구한 후 2를 곱하는 방법으로 시간 복잡도를 줄였고 1~n 의 합은 n(n+1)/2로 구했습니다.

my solving

c++

#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
typedef long long ll;
 
 
int getNumDivisors(ll num)
{
    int ret = 0;
    ll sqrtn = sqrt(num);
    for (ll i = 1; i <= sqrtn; i++)
    {
        if (num % i == 0)
            ret++;
    }
 
    if (num > 1)
        ret *= 2;
 
    return ret;
}
 
 
 
int main() 
{
    ll ret = 0;
    for (ll i = 1; ; i++)
    {
        int triangle = i * (i + 1) / 2;
 
        if (getNumDivisors(triangle) >= 500)
        {
            ret = triangle;
            break;
        }
    }
 
    cout << ret << endl;
 
    system("pause");
    return 0;
}