Sum of Digits Until a Single Digit [Solution]

Here’s a Python implementation of the add_digits function:

def add_digits(num: int) -> int:
    if num == 0:
        return 0
    elif num % 9 == 0:
        return 9
    else:
        return num % 9

# Example usage:
result = add_digits(123)
print(result)  # Output: 6 since 1 + 2 + 3 = 6.

result = add_digits(38)
print(result)  # Output: 2 since 3 + 8 = 11 and 1 + 1 = 2.

result = add_digits(9875)
print(result)  # Output: 2 since 9 + 8 + 7 + 5 = 29 and 2 + 9 = 11 and 1 + 1 = 2.

result = add_digits(1)
print(result)  # Output: 1. 

This solution takes advantage of the mathematical pattern where the result is equal to the remainder of the number divided by 9, except when the number is divisible by 9. In that case, the result is 9.

Time Complexity:

The time complexity is O(1),

Space Complexity:

The space complexity is O(1).