题解 | #规律数列求和#

#include <iostream>
using namespace std;

int main() {

    long int sum = 0;
    long int  n = 10;

    // //用数组
    // long int a[] = {9, 99, 999, 9999, 99999, 999999, 9999999, 99999999, 999999999, 9999999999};
    // for (int i = 0; i < 10; i++) {
    //     sum = sum + a[i];
    // }

    //用循环
    while(n < 100000000000 ) {
        sum = sum + (n - 1) ;
        n = n * 10;
    }

    cout << sum << endl;

    return 0;
}