def METHOD_NAME(self):
    """
    Search for a winner. Return None if game isn't over, "X" if player X won,
    "O" if player O won, and the empty string if it's a draw.
    """
    draw = True
    for line in LINES:
        c1 = self.cells[line[0]]
        if c1:
            c2 = self.cells[line[1]]
            c3 = self.cells[line[2]]
            if c1 == c2 and c1 == c3:
                return c1
            if (not c2) or (not c3):
                draw = False
        else:
            draw = False
    if draw:
        return ''