Return the shortest substring of text containing every character of pattern with its multiplicity. Choose the earliest on a tie. Empty pattern or no cover returns an empty string.

from collections import Counter
needed = Counter('AAB')
needed['A'] -= 1
print(needed['A'])  # one A still missing

Count missing character occurrences, expand until covered, then shrink while valid. Updating the deficit before moving the left edge preserves duplicate requirements. Before writing code, identify what each variable means after one iteration. Walk through the example and explain why the next transition preserves that meaning. Then check the smallest permitted input and a case with repeated values. A correct answer follows the contract for every input, including the boundary cases in the checks. Keep any input mutation within the stated rules; return a fresh result when the task asks for one.

Return the shortest substring of text containing every character of pattern with its multiplicity. Choose the earliest on a tie. Empty pattern or no cover returns an empty string.

The explanation opens here once every check passes.