1. A method for processing an input through each of a plurality of layers of a neural network to generate an output, wherein each of the plurality of layers of the neural network comprises a respective plurality of nodes, the method comprising, for a particular layer of the plurality of layers: receiving, by a classification system, an activation vector as input for the particular layer; selecting one or more nodes in the particular layer using the activation vector and a hash table that maps numeric values to nodes in the particular layer; and processing the activation vector using the selected nodes to generate an output for the particular layer.