add Digits Algorithm

A check digit is a form of redundancy check used for mistake detection on identification numbers, such as bank account numbers, which are used in an application where they will at least sometimes be input manually. With a check digit, one can detect simple mistakes in the input of a series of characters (normally digits) such as a individual mistyped digit or some permutations of two successive digits.

add Digits source code, pseudocode and analysis

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