Implement Queue by Two Stacks

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

The queue should supportpush(element),pop()andtop()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

public class Queue {
    private Stack<Integer> stack1;
    private Stack<Integer> backup;

    public Queue() {
       // do initialization if necessary
       stack1 = new Stack<Integer>();
       backup = new Stack<Integer>();
    }

    public void push(int element) {
        // write your code here
        while(!stack1.isEmpty()) {
            backup.push(stack1.pop());
        }
        stack1.push(element);
        while(!backup.isEmpty()) {
            stack1.push(backup.pop());
        }
    }

    public int pop() {
        // write your code here
        return stack1.pop();
    }

    public int top() {
        // write your code here
        return stack1.peek();
    }
}