Problem
Sometimes a program requires two stacks containing the same type of entries. two coexisting stacks If the two stacks are stored in separate arrays, then one stack might overflow while there was considerable unused space in the other. A neat way to avoid this problem is to put all the space in one array and let one stack grow from one end of the array and the other stack start at the other end and grow in the opposite direction, toward the first stack. In this way, if one stack turns out to be large and the other small, then they will still both fit, and there will be no overflow until all the space is actually used. Define a new class Double_stack that includes (as private data members) the array and the two indices top_a and top_b, and write code for the methods Double_stack( ), push_a( ), push_b( ), pop_a( ), and pop_b( ) to handle the two stacks within one Double_stack.
