Solve Ubuntu 12.04/14.04 can not login to desktop

Reason why is lightdm or ubuntu-desktop damage

1. install gdm
after startup press ctrl+alt+F1 and login
apt-get -f install gdm
dpkg-reconfigure gdm  -> select gdm
reboot

2.
After startup
gdm simple mode login
open terminal
(If terminal all black or all white then setting forground color
Ternimal -> 編輯 -> 設定組合偏好設定 -> 顏色 -> 文字顏色 -> 白色)

3.
apt-get -f install lightdm
apt-get -f install ubuntu-desktop
dpkg-reconfigure gdm  -> select lightdm
reboot

4.
if depends(相依性問題) happened, [aptitude] will handle it
apt-get install aptitude
aptitude -f install lightdm
aptitude -f install ubuntu-desktop
reboot

5. (option)
if aptitude fail, it's mean you already installed another version of element
(如果還是沒有安裝 那表示相依關係中有些元件已安裝過 但版本不合)
then
apt-get purge [installed element]
//again
aptitude -f install lightdm
aptitude -f install ubuntu-desktop
reboot