General idea is regular search with keeping node stack.
If node is found, then return current node.
If node is htable, search for suffixes in htable.
If anything fails, walk up the visited nodes and check if
any visited node has value. If yes, then it is a prefix.

f.e. searching for 'abcd'
visited nodes: root -a-> {1:trie} -b-> {2:htable}
1. search htable for 'cd', 'c'
2. retrace to node {1}, does it have value?
3. yes, node1 is prefix 'ab' for 'abcd'

Change-Id: I622adbe9a127d8317b933af4bf56e1411e2d915f