How can I add the digits of my input ysing array?

Question

Like if I input 42 the answer will be 6 ( 4 + 2 ).

Answer 1

Without array:

int n, sum;
cin >> n;
sum = 0;
while (n > 0) {
sum += n % 10;
n /= 10;
}
cout << sum << endl;

With array:

int i, n, sum;
int digits[16] = {0};
cin >> n;
i = 0;
while (n > 0) {
digits[i] = n % 10;
i++;
n /= 10;
}
digits[i] = -1;
sum = 0;
i = 0;
while (digits[i] != -1) {
sum += digits[i];
i++;
}
cout << sum << endl;

Answer 2

how do I start if it's like this:

int sumDigits(int n)
{

}