Find shortest substring that contains all the letters in the dictionary

"""
given a string and dictionary find the shortest substring containing all chars in dictionary
"""

from collections import defaultdict

def find_min_window(S: str, T: list[str]):
    left, right = 0, 0
    c = defaultdict(int) # frequency count for T

    for ch in T:
        c[ch] += 1

    min_str = None

    while right <= len(S):
        if all(x <= 0 for x in c.values()):
            if min_str is None or right - left <= len(min_str):
                min_str = S[left:right]

            if S[left] in c:
                c[S[left]] += 1
            left += 1
        else:
            if right == len(S):
                break

            if S[right] in c:
                c[S[right]] -= 1
            right += 1

    return min_str if min_str else ''