如何通过连接所有字符串但保留对象来折叠数组

def fold_array(arr):
    if isinstance(arr, list):
        result = []
        for item in arr:
            if isinstance(item, str):
                result.append(item)
            elif isinstance(item, list):
                result.extend(fold_array(item))
            else:
                raise ValueError("Invalid input: array contains non-string and non-array objects")
        return result
    elif isinstance(arr, str):
        return [arr]
    else:
        raise ValueError("Invalid input: input is not a string or an array")

# 示例用法
input_array = ["a", ["b", "c"], "d", [["e"], "f"]]
output_array = fold_array(input_array)
print(output_array)