题解 | 规律数列求和

#include<iostream>
using namespace std;
int main()
{
    long long arr[10];
    arr[0] = 9;
    long long sum = arr[0];
    for (int i = 1; i < 10; i++)
    {
        arr[i] = arr[i - 1] * 10 + 9;
        sum += arr[i];
    }
    cout << sum << endl;
    return 0;
}