包含我需要的所有符号的字符串的最小片段

def find_smallest_substring(target_str: str) -> str:
    symbols = set(target_str)  # 存储目标字符串中的符号
    symbol_count = len(symbols)  # 目标字符串中符号的总数
    window_start = 0  # 滑动窗口的起始位置
    window_len = float('inf')  # 最小片段长度
    min_start = 0  # 最小片段起始位置
    current_symbols = {}  # 当前滑动窗口中的符号

    for window_end in range(len(target_str)):
        symbol = target_str[window_end]
        current_symbols[symbol] = current_symbols.get(symbol, 0) + 1

        while len(current_symbols) == symbol_count:
            # 更新最小片段长度和最小片段起始位置
            if window_end - window_start + 1 < window_len:
                window_len = window_end - window_start + 1
                min_start = window_start

            # 移除滑动窗口起始位置的符号，并更新符号计数
            symbol_at_start = target_str[window_start]
            current_symbols[symbol_at_start] -= 1
            if current_symbols[symbol_at_start] == 0:
                del current_symbols[symbol_at_start]
            window_start += 1

    return target_str[min_start:min_start+window_len]

target_str = input("请输入目标字符串：")
smallest_substring = find_smallest_substring(target_str)
print("包含所有符号的最小片段为：", smallest_substring)