$8\mathrm{.}9$ The input stream of a stack is a list of all the elements we pushed onto

the stack, in the order that we pushed them. The input stream from

Exercise $8\mathrm{.}8$ was ABCDEFGHIJKLM

Exercises $305$

The output stream is a list of all the elements that are popped off the

stack, in the order that they are popped off.

a$.$ What is the output stream from Exercise $8\mathrm{.}8?$

Hint: BDE $...$

b$.$ If the input stream is ZYXWVUTSR, create a sequence of pushes and

pops such that the output stream is YXVUWZSRT.

c$.$ If the input stream is ZYXW$,$ how many different output streams can

be created?