题解 | #用两个栈实现队列#

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()) {
            if(stack1.isEmpty()) {
                throw new Exception("no elements");                
            } else {
                while(!stack1.isEmpty()) {
                    stack2.push(stack1.pop());
                }
            }
        }
        return stack2.pop();
    }
}