MJ [44] Find the Longest Substring

Question:
Give a dictionary "dict" and a string "S", find the longest valid word in dict which is a substring of "S".
Eg. dict = {"abc", "defgh", "ef"}
       S = "adbecfgh".
It should return "defgh".

Ref
[1] http://www.mitbbs.com/article_t/JobHunting/32960525.html