Maybe not as clean but more space efficient?

In the visual representation of the algorithm for the answer, the number 4 repeats twice.

Solution without splitting. this is more proper

I didn't pay enough attention to the description so I though it's required to use only stack operations, without any splitting processing char by char

Use an ArrayDeque so that the code leverages modern Java API

Creating a string part uses FIFO instead of LIFO

Using stack as array is cheating :P