Added doubly linked list and helloworld.
Given:You have given Amazon's stock prices for next 10 days
Find out: max benefit in best time complexity to buy and sell 1 share
Condition: Share must be sold any day after buying date.

For ex:
Share in thousands

5 1 4 6 7 8 4 3 7 9
Max benefit 9 - 1 = 8 thousand