题解 | #规律数列求和#

方法一：

#include using namespace std;

int main() {

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

cout << sum << endl;

return 0;

}

方法二：

#include using namespace std;

int main() {

long long a = 9;
long long sum = 0;

for(int i = 0; i < 10;  i++){
 if(i == 0){
     sum = a;
 }
 else{
     a = a * 10 + 9;
     sum = sum + a;
 }
}
cout << sum << endl;
return 0;

}