Stack

Consider the following sequence of operations on an empty stack.

push(54); push(52); pop(); push(55); push(62); s=pop();

Consider the following sequence of operations on an empty queue.

enqueue(21); enqueue(24); dequeue(); enqueue(28); enqueue(32); q=dequeue();

The value of s+q is ___________.

A stack is implemented with an array of { }^{\prime} A[0 \ldots N-1]^{\prime} and a variable \text { 'pos'. } The push and pop operations are defined by the following code.

 push (x)
    A[pos] <- x
    pos <- pos -1
end push
pop()
    pos <- pos+1
    return A[pos]
end pop

Which of the following will initialize an empty stack with capacity N for the above implementation?

Convert the pre-fix expression to in-fix -^{*}+A B C^{*}-D E+F G

Choose the equivalent prefix form of the following expression
(a+(b-c))^{\star}((d-e) /(f+g-h))

The best data structure to check whether an arithmetic expression has balanced parenthesis is a: