This is my favorite and I think the more simple  if you want permanent redirect  from NON WWW to WWW insert the following code in the nginx configuration file nginx.conf :

server {
  listen 80;
  server_name www.debian-tutorials.com;
  rewrite ^/(.*)$ http://debian-tutorials.com/$1 permanent;
}
Permanent redirect from WWW to NON WWW in nginx, use this code in your nginx.conf :
server {
  listen 80;
  server_name debian-tutorials.com;
  rewrite ^/(.*)$ http://www.debian-tutorials.com/$1 permanent;
}

Enjoy!