剑指offer牛客--用两个栈实现队列

要考虑到队列可能为空情况，抛异常。这样才全面。 

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())
            return stack2.pop();
        while(!stack1.isEmpty()){
            stack2.push(stack1.pop());
        }
        if(stack2.isEmpty())
            throw new RuntimeException("the queue is empty");
        return stack2.pop();
        
    }
}