用2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) {
        if (stack1.isEmpty()) {
            stack1.push(node);
        } else {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
            stack1.push(node);
            while (!stack2.isEmpty()) {
                stack1.push(stack2.pop());
            }
        }
    }
    
    public int pop() {
        //这里注意下返回值为null的情况，不能转为int
        return stack1.pop();
    }
}