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def min_window_substring(s: str, t: str) -> str:
    # L6 Staff Engineer Solution: Two Pointers + Sliding Window
    from collections import Counter
    if not t or not s: return ""
    target_counts = Counter(t)
    required = len(target_counts)
    l, r, formed = 0, 0, 0
    window_counts = {}
    ans = float("inf"), None, None

    while r < len(s):
        char = s[r]
        window_counts[char] = window_counts.get(char, 0) + 1
        if char in target_counts and window_counts[char] == target_counts[char]:
            formed += 1
        while l <= r and formed == required:
            char = s[l]
            if r - l + 1 < ans[0]:
                ans = (r - l + 1, l, r)
            window_counts[char] -= 1
            if char in target_counts and window_counts[char] < target_counts[char]:
                formed -= 1
            l += 1
        r += 1
    return "" if ans[0] == float("inf") else s[ans[1] : ans[2] + 1]