Credit Card Validator

You need to verify if the given credit card number is valid. For that you need to use the Luhn test.

Here is the Luhn formula:

1. Reverse the number.
2. Multiple every second digit by 2.
3. Subtract 9 from all numbers higher than 9.
4. Add all the digits together.
5. Modulo 10 of that sum should be equal to 0.

Task

Given a credit card number, validate that it is valid using the Luhn test. Also, all valid cards must have exactly 16 digits.

Input Format

A string containing the credit card number you need to verify.

Output Format

A string: ‘valid’ in case the input is a valid credit card number (passes the Luhn test and is 16 digits long), or ‘not valid’, if it’s not.

Sample Input

4091131560563988

Sample Output

valid

Explanation

Let’s run the Luhn test for our input:

Reverse: 8893650651311904 Multiplying the even positions by 2: 8 16 9 6 6 10 0 12 5 2 3 2 1 18 0 8 Subtract 9 from digits > 9: 8 7 9 6 6 1 0 3 5 2 3 2 1 9 0 8 The sum: 70 Sum Modulo 10: 70 % 10 = 0. The input passed the Luhn test and contains 16 digits, making it a valid credit card number.

Solutions

Language	Solution
C	not available
CPP	not available
C#	not available
Python	available
Java	not available
Ruby	not available
Swift	not available