Pardon me, You're correct.

                   1234567890123
ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZ ?
     3210987654321

There is something wrong with the test cases that include 's' or 'S'.
's', after 13 positions shift, should equal 'h' instead of 'f'.
Analogously, 'S' should equal 'H' instead of 'F'.

Groovy Translation

There is no more effective way to solve this problem. At the very least, you would need to travese the string which takes O(N). For extra space complexity, the variable crypt to store each key-value pair has O(1) space complexity since its size is not dependent on the size of the input string. If not consdiering the output string to be returned, the space complexity would be O(1) as well.

you keep incrementing the str pointer. When you return it, it is pointing to the nul-terminator, so it's effectively an empty string. Also, you overflow the char datatype by adding 13 to the last ASCII letters (e.g. 'z' + 13)

"The one you linked is deciphering it, this one is encoding it, they're not the same."

...yes they are?
eg:

decode: "ABCD" -> "NOPQ"
encode: "ABCD" -> "NOPQ"

The encode function is exactly the same as the decode one..

It is already specified how they should be handled in description.

If there are numbers or special characters included in the string, they should be returned as they are

Anyone reading, feel free to continue discussing, but since this has nothing to do with improving the kata I'm marking this as resolved.

You are not able to nest functions?

it's no longer there

You are correct. I just messed the registers up. Have a nice day.