implement 2 stacks using 1 array

[ashokdey]

🏖 What's inside?

So I went ahead to add a new problem in the Stack section.

• The implementation is not regular because it does not deal with a fixed size array
• I doubt that I may not have covered all the edge cases
• In terms of optimization also, I feel there is a scope of improvement.

[ashokdey]

Since insertions and deletions are going O(n) which is breaking the rules of Stack, this implementation is invalid.