i got a crude formula and used mathway :P

OP solved it, closing

Inception

There's a brute force, recursive solution which will time out (it happened to me the first time) and then there are smart solutions, much more efficient.
I suggest using the brute force solution to print combinations that you can analyze and recognize patterns, which will then inspire you to come up with something more efficient.

W

It's not a mistake, it's a masterpiece

Ain't nothing but a mistake

Perfection.

Ain't nothing but a heartache

Allocation for new string is expensive.

just tell me why?

When a = "011" and b = "01",your solution compared them as 0 vs 0 and 1 vs 1 but it should be 0 vs 0, 0 vs 1 and 1 vs 1 (011 & 001)

It should be infinite, in other words - endless, limitless, illimitable, unending, unceasing, perpetual.
So never ending but within a time limit (12 seconds).

How to dislike this???

ahaahhahhah