def encode(num, alphabet):
    """Encode a positive number into Base X and return the string.

    Arguments:
    - `num`: The number to encode
    - `alphabet`: The alphabet to use for encoding
    """
    if num == 0:
        return alphabet[0]
    arr = []
    arr_append = arr.append  # Extract bound-method for faster access.
    _divmod = divmod  # Access to locals is faster.
    base = len(alphabet)
    while num:
        num, rem = _divmod(num, base)
        arr_append(alphabet[rem])
    arr.reverse()
    return "".join(arr)