This is a *medium* level problem with two levels of difficulty.

```
Input Description
-----------------
On standard console input, you will be given an array of digits (0 to 9) and
spaces. All digits will be space-delimited, unless the digits represent
multiple presses of the same button (for example pressing 2 twice gives you the
letter 'B'). Use the modern Telephone Keypads digit-letter layout:
0 = Not used
1 = Not used
2 = ABC
3 = DEF
4 = GHI
5 = JKL
6 = MNO
7 = PQRS
8 = TUV
9 = WXYZ
You may use any source for looking up English-language words, though this
simple English-language dictionary is complete enough for the challenge.
Output Description
------------------
Print a list of all best-fitting words, meaning words that start with the word
generated using the given input on a telephone keypad. You do not have to only
print words of the same length as the input (e.g. even if the input is
4-digits, it's possible there are many long words that start with those
4-digits).
```

You could do this by reading the dictionary into a vector of strings, then figuring out the prefix determined by the input and matching it against each dictionary word. There is a better way to do this though, which makes more sense considering the second part of this question.

Instead of using a vector, you could use a Trie which in this case would look something like:

(showing the words ‘sailor’, ‘soldier’ and ‘solder’)

{% img center http://farm6.staticflickr.com/5534/12005243014_69145160ae_o_d.jpg 400 %}

This is a sort of naive implementation, you could compress this by storing strings and only expanding out to character chains if needed, but I skipped that optimization here.

There is a harder version of this problem:

```
Challenge++
-----------
If you want an extra challenge, accomplish the same challenge but without
knowing the number of times a digit is pressed. For example "7653" could mean
sold, or poke, or even solenoid! You must do this efficiently with regards to
Big-O complexity.
```

This would have been more inefficient with a vector of strings (you would sort the vector and then for each prefix, print out all subsequent ones until you encounter a string with a different prefix. So it’s O(n log n) where n is the size of the dictionary).

With our trie, we can (once we have a list of all combinatorially possible prefixes) look them up *quickly* (O(k + n), where k is the length of the prefix; O(k) time for each lookup and O(n) time for creating the trie (or O(mn) where m is the length of the largest word, though I would consider that a constant here)).

(Note: there’s some duplication of common code, especially in the `Trie`

class; sorry about that!)