def swap_inds_helper(i, j, arr):
    """
    Swaps indices in array, in-place.

    :param int i: index 1
    :param int j: index 2
    :param array-like arr: {list, np.array} array to be modified in-place
    """
    tmp = arr[i]
    arr[i] = arr[j]
    arr[j] = tmp
