Design and implement a data structure for a compressed string iterator. The given compressed string will be in the form of each letter followed by a positive integer representing the number of this letter existing in the original uncompressed string.
Implement the StringIterator class:
next() Returns the next character if the original string still has uncompressed characters, otherwise returns a white space.
hasNext() Returns true if there is any letter needs to be uncompressed in the original string, otherwise returns false.