I might be naive comes to classic ML / NLP: how do you keep the prefix table for N = 5? Naively, that look-up table would be 100k^5 (assuming 100k vocabulary si

I might be naive comes to classic ML / NLP: how do you keep the prefix table for N = 5? Naively, that look-up table would be 100k^5 (assuming 100k vocabulary size). Is it very sparse? How large that usually is?