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。
Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.
如果该对象小于指定对象返回负数，等于返回0，大于返回正数。
Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
比较用来排序的两个参数。根据第一个参数小于、等于或大于第二个参数分别返回负整数、零或正整数。
Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
比较用来排序的两个参数。根据第一个参数小于、等于或大于第二个参数分别返回负整数、零或正整数。
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