Supervised learning for variable length feature-less data
This is a typical scenario in language processing, e.g. if you want to read a product review representing each word with a number and output a 1-5 star rating.

You can use a recurrent many-to-one RNN/GRU/LSTM network: After feeding each word it generates a feature vector which can be fed into a classifier.
https://en.wikipedia.org/wiki/Recurrent_neural_network