两栈实现一个队列

public class Solution {   
  Stack stack1 = new Stack();    
  Stack stack2 = new Stack();        
  public void push(int node) {        
  stack1.push(node);   
  }        
  public int pop() { 
      if(stack1.empty()&&stack2.empty()){  
         throw new RuntimeException("Queue is empty!");  
      }    
    if(stack2.empty()){       
     while(!stack1.empty()){  
              stack2.push(stack1.pop());   
         }     
   }     
   return stack2.pop();  
  } 
}