def dup_lshift(f, n, K):
    """
    Efficiently multiply ``f`` by ``x**n`` in ``K[x]``.

    **Examples**

    >>> from sympy.polys.domains import ZZ
    >>> from sympy.polys.densearith import dup_lshift

    >>> f = ZZ.map([1, 0, 1])

    >>> dup_lshift(f, 2, ZZ)
    [1, 0, 1, 0, 0]

    """
    if not f:
        return f
    else:
        return f + [K.zero]*n