[C++] 백준 11687번 : 팩토리얼 0의 개수

https://www.acmicpc.net/problem/11687

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

int countZero(int n)
{
    int cnt = 0;
    while (n >= 5)
    {

        cnt += n / 5;
        n /= 5;
    }
    return cnt;
}
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);

    int m;
    cin >> m;

    int bottom = 0, top = m * 5, ans = -1;

    while (bottom <= top)
    {
        int mid = (bottom + top) / 2;
        int tmp = countZero(mid);
        if (tmp < m)
        {
            bottom = mid + 1;
        }
        else
        {
            if (tmp == m)
                ans = mid;
            top = mid - 1;
        }
    }

    cout << ans;

    return 0;
}