Exact same as https://leetcode.com/discuss/interview-question/1028666/Coinbase-or-Phone-or-In-memory-File-System

Design an in memory file system with the folliowing functions:

mkdir: Either create exactly 1 directory, or throw exception
writeFile: write file to existing or new. dont write to directory location
readFile: read contents of file, dont read from directory location

Used a JS trie implementation. Passed the phone screen with this.