Recently, I lost my sudo privilege for linux system, which make me feel panic. I don't want to reinstall the system again. I got no time to do this.
However, I was lucky enough to find a solution.
In fact it is easy. Restart the system and select recovery mode from grub menu.
Next step, just select fsck so that you can mount / as a writeable directory.
When you finish the step above, select enter the system as root.
sudo addgroup your-username sudo
Now you are fine.
