import sys


def find_min(characters):
    min_i = sys.maxsize
    for ch, i in characters.items():
        if i < min_i:
            min_i = i
    return min_i


def find_max(characters):
    max_i = -1
    for ch, i in characters.items():
        if i > max_i:
            max_i = i
    return max_i


def min_substring(S, T):
    characters = {}
    for ch in T:
        if ch not in characters:
            characters[ch] = -1

    min_i = -1
    max_i = len(S)
    min_length = len(S) + 1
    matched = 0

    for i, ch in enumerate(S):
        if ch not in characters:
            continue

        if characters[ch] == -1:
            matched += 1

        characters[ch] = i

        if matched < len(characters):
            continue

        left_i = find_min(characters)
        right_i = find_max(characters)
        length = right_i - left_i
        if length < min_length:
            min_i = left_i
            max_i = right_i
            min_length = length

    return S[min_i:max_i + 1]


if __name__ == '__main__':
    print(min_substring('ADOBECODEBANC', 'ABC'))