604. Design Compressed String Iterator:

The Essence: Each letter of the string has a number on the right side of it. The number can be between 1 and 10^9. It is why we need to compute the digital value of this number first.

Details: After calculating the number, we can decrement the number at each time the next() method is called.

The problem-solver can find a straight-forward implementation here: https://github.com/spiralgo/algorithms/blob/master/src/main/java/algorithms/curated170/easy/DesignCompressedStringIterator.java