from typing import List


def get_neighbor_atom_indices(mol, atom_indices: List[int]=[]) -> List[int]:
    """
    Get indices of neighboring atoms

    Parameters
    ----------
    mol: RDKit.Chem.rdchem.Mol
        RDKit molecule
    atom_indices: list (optional)
        Atom indices to get the neighbors for

    Returns
    -------
    Indices of neighboring atoms
    """
    neighbor_indices = []
    for i in atom_indices:
        atom = mol.GetAtomWithIdx(i)
        for nbr in atom.GetNeighbors():
            neighbor_indices.append(nbr.GetIdx())
    return neighbor_indices