def METHOD_NAME(tile_coord):
    """
    >>> quadkey((0, 0, 1))
    '0'
    >>> quadkey((1, 0, 1))
    '1'
    >>> quadkey((1, 2, 2))
    '21'
    """
    x, y, z = tile_coord
    quadKey = ""
    for i in range(z,0,-1):
        digit = 0
        mask = 1 << (i-1)
        if (x & mask) != 0:
            digit += 1
        if (y & mask) != 0:
            digit += 2
        quadKey += str(digit)
    return quadKey