Add incremental hints for each challenge notebook

[Donne Martin]

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.

[basanherki]

hi how ru please see