#17 Letter Combinations of a Phone Number
A simple problem which illustrates how we can solve these kind of *all possible solutions* with Recursion(Backtrack) or Iteration(BFS).
In a Backtrack solution, just pass down the temporary data into next recursion call and just the temporary data agains our needs.
In a BFS situation, remember to add a ending identifier such as NULL to indicate that one Round is finished
and currently practicing leetCode online. If you are interested, maybe we can learn together.