题解 | 规律数列求和

#include <iostream>
using namespace std;

int main() {

    // write your code here......
    long long int sum = 0, mul = 1, total_sum = 0;
    for (int i = 1; i <= 10; i++){
        mul = 1;
        for(int j = 1; j <= i; j++){
            mul *= 10;
        }
        sum += mul / 10;
        total_sum += sum;
    }
    cout << total_sum * 9 << endl;

    return 0;
}