Correct Answer - Option 2 : 2 queues
A stack can be implemented using two queues. Let stack to be implemented be ‘x’ and queues used to implement be ‘a’ and ‘b’.
Method 1 (By push operation)
This method makes sure that the newly entered element is always at the front of ‘a’, so that pop operation just dequeues from ‘a’. ‘b’ is used to put every new element at front of ‘b’.
Method 2 (By making pop operation costly)
In a push operation, the new element is always enqueued to a. In pop() operation, if b is empty then all the elements except the last, are moved to b. Finally, the last element is dequeued from a and returned.
Therefore Option 2 is correct
