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Add Digits

Explore how to reduce any integer to a single digit by repeatedly summing its digits. Understand the problem constraints, and discover a formula-based method to solve it in constant time without using loops or recursion.

Statement

Given an integer num, repeatedly sum all of its digits until the resulting value is a single digit, then return that value.

Note: Could you solve this without any loop or recursion in O(1)O(1) ...