2018-10-22 Implement Queue by Two Stacks [M]

40. Implement Queue by Two Stacks
    LC: 232
    Similar problem: Implement Stack by Two Queues [E]

As the title described, you should only use two stacks to implement a queue's actions.

The queue should support push(element), pop() and top() where pop is pop the first(a.k.a front) element in the queue.

Both pop and top methods should return the value of first element.

Example
push(1)
pop() // return 1
push(2)
push(3)
top() // return 2
pop() // return 2
Challenge
implement it by two stacks, do not use any other data structure and push, pop and top should be O(1) by AVERAGE.