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The [Luhn algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers. The task is to check if a given string is valid. Validating a Number ------ Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed. As an example of a valid string, here is a fictitious Canadian Social Insurance Number. ``` 046 454 286 ``` The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling ``` _4_ 4_4 _8_ ``` If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling: ``` 086 858 276 ``` Then sum all of the digits ``` 0+8+6+8+5+8+2+7+6 = 50 ``` If the sum is evenly divisible by 10, then the number is valid. This number is valid! An example of an invalid Canadian SIN where we've changed the final digit ``` 046 454 287 ``` Double the second digits, starting from the right ``` 086 858 277 ``` Sum the digits ``` 0+8+6+8+5+8+2+7+7 = 51 ``` 51 is not evenly divisible by 10, so this number is not valid. ---- An example of an invalid credit card account ``` 8273 1232 7352 0569 ``` Double the second digits, starting from the right ``` 7253 2262 5312 0539 ``` Sum the digits ``` 7+2+5+3+2+2+5+2+5+3+1+2+0+5+3+9 = 57 ``` 57 is not evenly divisible by 10, so this number is not valid.
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