Given a non-negative integer, repeatedly add its digits until the result becomes a single-digit number.

Question

Accepted Answer

Approach:
The solution repeatedly extracts each digit using the modulo (%) operator and computes their sum. If the resulting sum contains more than one digit, the same process is repeated until a single-digit value is obtained.

This iterative approach is easy to understand and works efficiently for any non-negative integer without using additional data structures.

Java Solution:
```
public class AddDigits {

public static int addDigits(int number) {
        while (number >= 10) {
            int digitSum = 0;

while (number > 0) {
                digitSum += number % 10;
                number /= 10;
            }

number = digitSum;
        }

return number;
    }

public static void main(String[] args) {
        int number = 9875;

int result = addDigits(number);

System.out.println("Single Digit Result: " + result);
    }
}
```
Output:
Single Digit Result: 2

Time Complexity:
O(d × k), where d is the number of digits and k is the number of iterations required to reduce the number to a single digit. In practice, the number of iterations is very small.

Space Complexity:
O(1), as only a few variables are used irrespective of the input size.

Key Interview Points:
• The modulo (%) operator is commonly used to extract digits from a number.
• The algorithm repeatedly processes digits until a single-digit result is obtained.
• An optimized mathematical approach using the Digital Root concept can solve the problem in O(1) time using the formula:
  result = (number == 0) ? 0 : 1 + (number - 1) % 9