Write a class Double Stack Queue which implements the Queue interface. Its only fields are two Stacks, front and back. Insertion is implemented by pushing onto back, while deletion is implemented by...

Write a class Double Stack Queue which implements the Queue interface. Its only fields are two Stacks, front and back. Insertion is implemented by pushing onto back, while deletion is implemented by popping front. The challenge is to deal with the situation where front is empty, back is not, and we have to pop.