题解 | #规律数列求和#

#include <iostream>
#include<cmath>
using namespace std;

int main() {

    // write your code here......
    long long sum = 0;
    long long arr = 0;
    for(int i = 0;i<=9;i++)
    {
        //arr += (pow(10,i)*9);
        arr = arr*10+9;
        sum+=arr;
    }
    cout<<sum;

    return 0;
}