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 $O(1)$ runtime?

Constraints:

• $0 \leq$ num $\leq 2^{31} - 1$

Now, let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Implement your solution in the following coding playground.