//思路：用两个栈后入先出，一个栈出到另一个栈，再从另一个栈出，就形成了先入先出的队列
import java.util.Stack;

public class Solution {
    Stack<Integer> stack1 = new Stack<Integer>();
    Stack<Integer> stack2 = new Stack<Integer>();
    
    public void push(int node) {
        stack1.push(node);
    }
    
    public int pop() throws Exception{
        if(stack2.isEmpty()){
            while(!stack1.isEmpty())
                stack2.push(stack1.pop());
        }
        if(stack2.isEmpty())
            throw new Exception("Queue is empty!");
        return stack2.pop();
    }
}