Minimum Window Substring

Return the shortest contiguous substring of text containing every character of required with at least its required multiplicity. Ignore no characters: comparisons are case-sensitive. If shortest windows tie, choose the smallest starting index. Return '' if required is empty or no window exists. Strings have at most 500 characters. Examples (JSON; null means Python None): Input: {"text": "ADOBECODEBANC", "required": "ABC"} Output: "BANC" Input: {"text": "aa", "required": "aa"} Output: "aa"

def minimum_window(text: str, required: str) -> str:
    """Find the earliest minimum window covering a character multiset."""
    pass