33_QueueByStack.java

public class Queue {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;

    public Queue() {
       // 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.empty()){
            if(stack1.empty())
                return 0;
            else{
                while(!stack1.empty()){
                    stack2.push(stack1.pop());
                }
            }
        }
        return stack2.pop();
    }

    public int top() {
        // write your code here
        if(stack2.empty()){
            if(stack1.empty())
                return 0;
            else{
                while(!stack1.empty()){
                    stack2.push(stack1.pop());
                }
            }
        }
        return stack2.peek();
    }
}