Lucky Digit

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We define "lucky digit" of an integer x using the following rules: If x has only 1 digit, then its lucky digit is x. Otherwise, the lucky digit of x is equal to the lucky digit of the digit-sum of x. Here, digit-sum of a number is defined as the sum of its digits. For example, lucky digit of 9875 will be calculated as: lucky-digit(5879) = lucky-digit(5+8+7+9) = lucky-digit(29) = lucky-digit(

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solution.cppC++17

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