Improving ForkJoin usage for better performance

Using this modification, there is no need to create new List<P> problems instances during the ForkJoin recursive operation. It will save you some memory, execution time, and energy consumption.
Merge pull request #1 from gustavopinto/master

Improving ForkJoin usage for better performance. There is no need to create new List<P> problems instances during the ForkJoin recursive operation. It will save you some memory, execution time, and energy consumption. Contributed by - @gustavopinto