5、用两个栈实现队列

用两个栈来实现一个队列，完成队列的Push和Pop操作。 队列中的元素为int类型。

=================Python===============

# -*- coding:utf-8 -*-
class Solution:
    def __init__(self):
        self.s1 = []
        self.s2 = []
        
    def push(self, node):
        # write code here
        self.s1.append(node)
        
    def pop(self):
        # return xx
        if len(self.s2) == 0:
            while self.s1:
                self.s2.append(self.s1.pop())
        return self.s2.pop()

====================Java=================

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.add(node);
    }
    
    public int pop() throws Exception {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.add(stack1.pop());
            }
        }
        if (stack2.isEmpty())
            throw new Exception("error");
        return stack2.pop();
    }
}