## The challenge

Let’s implement the Luhn Algorithm, which is used to help validate credit card numbers.

Given a positive integer of up to 16 digits, return `true` if it is a valid credit card number, and `false` if it is not.

Here is the algorithm:

• Double every other digit, scanning from right to left, starting from the second digit (from the right).Another way to think about it is: if there are an even number of digits, double every other digit starting with the first; if there are an odd number of digits, double every other digit starting with the second:`1714 ==> [1*, 7, 1*, 4] ==> [2, 7, 2, 4] 12345 ==> [1, 2*, 3, 4*, 5] ==> [1, 4, 3, 8, 5] 891 ==> [8, 9*, 1] ==> [8, 18, 1]`
• If a resulting number is greater than `9`, replace it with the sum of its own digits (which is the same as subtracting `9` from it):`[8, 18*, 1] ==> [8, (1+8), 1] ==> [8, 9, 1] or: [8, 18*, 1] ==> [8, (18-9), 1] ==> [8, 9, 1]`
• Sum all of the final digits:`[8, 9, 1] ==> 8 + 9 + 1 = 18`
• Finally, take that sum and divide it by `10`. If the remainder equals zero, the original credit card number is valid. `18 (modulus) 10 ==> 8 , which is not equal to 0, so this is not a valid credit card number`

## The solution in Java code

Option 1:

 `````` 1 2 3 4 5 6 7 8 9 10 11 `````` ``````public class Validate { public static boolean validate(String n) { final boolean[] flag = {(n.length() & 1) == 1}; return Arrays.stream( n.split("")) .map(Integer::parseInt) .mapToInt(value -> value) .map(integer -> ((flag[0] ^= true) ? (integer * 2 - 1) % 9 + 1 : integer)) .sum() % 10 == 0; } } ``````

Option 2:

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 `````` ``````public class Validate { public static boolean validate(final String n){ if (n == null || n.isEmpty()) return false; boolean x = true; int sum = 0; int temp = 0; for (int i = n.length() - 1; i >= 0; i--) { temp = n.charAt(i) - '0'; sum += (x = !x) ? temp > 4 ? temp * 2 - 9 : temp * 2 : temp; } return sum % 10 == 0; } } ``````

Option 3:

 ``````1 2 3 4 5 6 7 8 9 `````` ``````public class Validate{ public static boolean validate(String n){ boolean[] flag = { (n.length() & 1) == 1 }; return n.chars() .map(c -> c - '0') .map(num -> (flag[0] ^= true) ? (num * 2 - 1) % 9 + 1 : num) .sum() % 10 == 0; } } ``````

## Test cases to validate our solution

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 `````` ``````import org.junit.Test; import static org.junit.Assert.assertEquals; public class CreditcardValidationTest{ @Test public void test891(){ assertEquals(false, Validate.validate("891")); } @Test public void test123(){ assertEquals(false, Validate.validate("123")); } @Test public void test1(){ assertEquals(false, Validate.validate("1")); } @Test public void test2121(){ assertEquals(true, Validate.validate("2121")); } @Test public void test1230(){ assertEquals(true, Validate.validate("1230")); } @Test public void test8675309(){ assertEquals(false, Validate.validate("8675309")); } @Test public void test4111111111111111(){ assertEquals(true, Validate.validate("4111111111111111")); } @Test public void test26(){ assertEquals(true, Validate.validate("26")); } @Test public void test2626262626262626(){ assertEquals(true, Validate.validate("2626262626262626")); } @Test public void test91(){ assertEquals(true, Validate.validate("91")); } @Test public void test92(){ assertEquals(false, Validate.validate("92")); } @Test public void test912030(){ assertEquals(true, Validate.validate("912030")); } @Test public void test922030(){ assertEquals(false, Validate.validate("922030")); } } ``````