def find_neighbor(used_letters):
    """
    This function help find the current position's neighbors that have not been used previously.
    :param used_letters: list, list of the indexes of used letters
    :return: list, list of indexes that are current position's neighbors and not have been used
    """
    neighbor = []
    center = used_letters[len(used_letters)-1]   # Current position
    # x-index and y-index of current position
    x = int(center[1])
    y = int(center[0])
    # Find neighbors
    for i in range(x-1, x+2):
        if 0 <= i <= 3:
            for j in range(y-1, y+2):
                if 0 <= j <= 3:
                    index = str(j) + str(i)
                    if index not in used_letters:
                        neighbor.append(index)
    return neighbor