题解 | #规律数列求和#

#include <iostream>
using namespace std;

int main() {

    // write your code here......
    long sum = 0;
    long a[10] = {9, 99, 999, 9999, 99999, 999999, 9999999, 99999999, 999999999, 9999999999};

    for (int i = 0; i < 10; i++) {
        sum += a[i];
    }
    cout << sum << endl;

    return 0;
}

#include <iostream>
using namespace std;

int main() {

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

    for (int i = 1; i <= 10; i++) {
        num = num * 10 + 9;
        sum += num;
    }
    cout << sum << endl;

    return 0;
}