Add incremental hints for each challenge notebook

Hints could be provided to help users incrementally arrive at the desired solution. Users could %load these hints into the **Challenge Notebook** when needed.
## Hints

Run the following cell(s) for a hint:

```
%load hint1.txt
```

```
%load hint2.txt
```

Running the cell(s) would would load the contents of the accompanying hint file:

```
# %load hint1.txt
Consider a brute force solution that scans each
element and compares it with every other element.
```

```
# %load hint2.txt
The brute force solution has a time complexity of O(n^2).
A hash map lookup could help make this linear.
```