Add Digital Root algorithm to maths (#14988)

* Add digital root algorithm

* refactor: use descriptive parameter name `number` instead of `n` (algorithms-keeper)

Also see: project_euler/problem_092/sol1.py

maths/digital_root.py

@@ -0,0 +1,46 @@
+from __future__ import annotations
+
+"""
+Digital Root.
+The digital root (also known as the repeated digital sum) of a non-negative
+integer is the (single digit) value obtained by an iterative process of summing
+digits: on each iteration the digits of the previous result are summed, and the
+process continues until a single-digit number is reached.
+
+For more information see:
+https://en.wikipedia.org/wiki/Digital_root
+"""
+
+
+def digital_root(number: int) -> int:
+    """
+    Return the digital root of a non-negative integer number.
+
+    The digital root is the single-digit value obtained by repeatedly summing
+    the decimal digits of number until only one digit remains. The input is taken by
+    its absolute value, so negative numbers behave like their positive
+    counterpart.
+
+    >>> digital_root(0)
+    0
+    >>> digital_root(9)
+    9
+    >>> digital_root(38)
+    2
+    >>> digital_root(12345)
+    6
+    >>> digital_root(-45)
+    9
+    >>> digital_root(999999999999)
+    9
+    """
+    number = abs(number)
+    while number >= 10:
+        number = sum(int(digit) for digit in str(number))
+    return number
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()