You are using the following algorithm to implement a queue by reusing two stacks.
A(q, x)
1) Push x to stack1
B(q)
1) If both stacks are empty then error.
2) If stack2 is empty
While stack1 is not empty, X
3) Pop the element from stack2 and return it.
What can be used in place of X to complete the algorithm?
[Assume that the size of stacks is unlimited.]
