# Assignment 2: Stack permutations

### (Submit at moodle.)

In this programming we have two queues, the input queue Q1 and the output queue Q2, and one stack S. We are only allowed to dequeue from Q1 and we are only allowed to enqueue into the output queue Q2. We are allowed to both push into and pop from the stack.

Consider that we have the numbers 1 2 3 4 5 6 enqueued in Q1. Suppose we perform the following sequence of operations.

enqueue(Q2, dequeue(Q1))
push(S, dequeue(Q1))
enqueue(Q2, dequeue(Q1))
push(S, dequeue(Q1))
enqueue(Q2, pop(S))
enqueue(Q2, pop(S))
enqueue(Q2, dequeue(Q1))
enqueue(Q2, dequeue(Q1))

then the output queue contains 1 3 4 2 5 6

This is an example of what we call a *stack permutation* i.e. A stack permutation is a ordering of numbers from 1 to n that can be obtained from the initial ordering 1, 2, ... n by a sequence of stack operations as described above.

To clarify this, note that 1 5 3 4 2 6 is *not* a stack permutation. Intuitively this is because to enqueue 5 into Q2 after 1 we would have to push 2 3 4 into the stack which would then be output in the order 4 3 2, not in the order 3 4 2.

In this assignment you will be given a permutation of n numbers and asked to check if it is a stack permutation or not. The TA will give you the number n as input and will give you a permutation. If it is a stack permutation your program will have to return the sequence of operations that formed that permutation. If it is not a permutation, your program will have to say it is not a permutation, and will have to give the reason why (as given above).

### NOTE:

• The honor code must be follow for all assignments
• The program should work on linux with java 1.6 (You could test it on one of the computer center machines or one of the CSE general computing lab machines.)