Implement Queue by Two Stacks

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.

Have you met this question in a real interview? 

Yes

Example

For push(1), pop(), push(2), push(3), top(), pop(), you should return 1, 2and 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.

public class Solution {

    private Stack<Integer> stack1;

    private Stack<Integer> stack2;

    public Solution() {

       // do initialization if necessary

      stack1 = new Stack<Integer>();

      stack2 = new Stack<Integer>();

    }

    

    public void push(int element) {

        // write your code here

        stack1.push(element);

    }

    public int pop() {

        // write your code here

        if(stack2.isEmpty()){

            getInorderStack2();

        }

        return stack2.pop();

    }

    public int top() {

        // write your code here

        if(stack2.isEmpty()){

            getInorderStack2();

        }

        return stack2.peek();

    }

    private void getInorderStack2(){

        while(!stack1.isEmpty()){

            stack2.push(stack1.pop());

        }

    }

}