Write a Python script that takes a string like '3a1b4j' and outputs 'aaabbjjj' by sorting the string and counting consecutive characters based on the numeric values.

The input format is a sequence of digit–letter pairs. We can parse the string, sort the pairs by letter, and then expand each letter according to its count:

import re

def expand(encoded: str) -> str:
    # Find all digit-letter pairs
    pairs = re.findall(r'(\d+)([a-zA-Z])', encoded)
    # Convert to list of (letter, count)
    items = [(letter, int(count)) for count, letter in pairs]
    # Sort by letter
    items.sort(key=lambda x: x[0])
    # Build result
    return ''.join(letter * count for letter, count in items)

print(expand('3a1b4j'))  # aaabbjjj

The regular expression captures each number and the following letter. After sorting alphabetically, we multiply each letter by its numeric count to produce the expanded string. This solution runs in O(n log n) time due to the sort, where n is the number of distinct letters.