题解 | #规律数列求和#

#include <iostream>
using namespace std;

int main() {

    // write your code here......
    long long a = 0;
    long long c = 0;
    for (int i = 1;i <= 10;i++){
        c = c * 10 + 9;
        a += c;
    }
    cout << a << endl;

    return 0;
}