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Given a number determine whether or not it is valid per the Luhn formula. 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. ## Example 1: valid credit card number ``` 4539 1488 0343 6467 ``` The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling ``` 4_3_ 1_8_ 0_4_ 6_6_ ``` If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling: ``` 8569 2478 0383 3437 ``` Then sum all of the digits: ``` 8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80 ``` If the sum is evenly divisible by 10, then the number is valid. This number is valid! ## Example 2: invalid credit card number ``` 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+6+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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