题解 | #规律数列求和#

#include <iostream>
using namespace std;

int main() {

    // write your code here......
    long sum = 0;
    long num = 0;
    long i;

    for (i = 1; i <= 10; i++) {
        num = num * 10 + 9;
        sum += num;

    }

    cout << sum << endl;

    return 0;
}