Added reference to Hacker Rank Data Structure Stacks problem Maximum Element.
Added reference to Hacker Rank Data Structure Stacks problem Maximum Element.

shivkrthakur/HackerRankSolutions,shivkrthakur/HackerRankSolutions,shivkrthakur/HackerRankSolutions,shivkrthakur/HackerRankSolutions