Exercise Idea : Problems involving techniques like sliding window/hashing

Hi there,

I am interested in contributing some exercises to this repo (exercises that can be done in many languages). Some ideas I have area:

1. Maximum contiguous array sum
2. Find whether a sum of 2 number exist
3. Find a contiguous array block that sums to a specific number

If possible, how should I go about adding them? I dont mind coming up with test cases to supplement them as well.