There are many similarities between 'A Clockwork Orange' and the film 'Leon'. Both main characters 'Alex' and 'Leon' are seen drinking milk many times in the films. Stansfeild (Leon) and Alex (ACO) both listen to Beethoven to stimulate their violence and the song 'Singing in the Rain' is featured in both films.

In the shot of the outside of Tony's restaurant, before the scene where we see him celebrating his birthday and being interrupted by Stansfield, you can see someone walking past the restaurant. That that person is Luc Besson, the director of the movie.