[C++]

Assuming that we enter the integer value is 5689798. Summing up the digits gives 5+6+8+9+7+9+8+8 = 58 that contains more than one digit, so we repeat the process: 5+8 = 13. we repeat until we got one digit: 1+3 = 4 . The final answer is 4. I don't know how to isolate each digit in order to add...