non-negative integer 非负整数
non-negative integer solution 非负整数解
Age is an integer, and must be non-negative.
年龄是整数，并且必须是非负数。
In the second line, there is one non-negative integer which is the number of questions asked and answers to them.
第二行是一个不大于五千的非负整数，表示你提问的次数。
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。
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