Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

给定一个非负整数，将该整数的所有数字的和作为新的整数，重复直至得到只有一个数的整数。

youdao

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

给定一个非负整数，将该整数的所有数字的和作为新的整数，重复直至得到只有一个数的整数。

youdao