My first sliding window problem. 
Sliding Window is an algorithmic technique used to process a contiguous subarray or substring by maintaining a moving window over the input, where thewindow expands and/or shrinks while traversing the data to efficiently compute results in linear time.

When to use Sliding Window :

>Subarray / substring problems

>Problems asking for:

1)longest / shortest window

2)maximum / minimum sum

3)unique elements in a range

4)Continuous range constraints