installing lightdm on linuxmint 18.2 causes a black screen at login

if you upgrade your distro to 18.2 and install lightdm and remove mdm and reboot you can get a black screen with no login option.

First, install LightDM by typing the following commands in a terminal:
apt install slick-greeter lightdm lightdm-settings apparmor
When asked to select a default display manager, select LightDM.
Then, remove MDM and reboot by typing the following commands in a terminal:
apt remove mdm
sudo reboot

i first reinstalled mdm as i could not find any solution. that helped get into the login.
but then on reboot startx kept telling me to choose between lighdm and mdm.

rebooting alt+ctrl+del and choose safe more and login into root . then
you need to run sudo dpkg-reconfigure lightdm

google search with their shitty ai was no help . how to reboot. especially for a novice linux user. 

finally tried some 2012 solution. and it worked