Here is the link.
The interviewer was very experienced, and he said that I know that you are experienced. You have 10 minutes for the first question, and then you have 20 minutes for the second algorithm.
I did not come out using Trie to put all substrings. Once the interviewer gave me hint to think about a string is 100 page long, a billion words. I quickly came out the idea to put all those Trie with substrings starting from 'a' to 'z'.