用两个栈实现队列

左思右想不会呀，结果嘞 看了答案焕然大悟。下面这个代码是错误的的   请问哪里错误了

class Solution
{
public:
    void push(int node) {
        stack1.push(node);
    }

    int pop() {
        int a ;
        if(stack2.empty() == true){
            while(stack1.empty() != true){
                a = stack1.top();
                stack2.push(a);
                stack1.pop();
            }
        }
        return stack2.top();
        stack2.pop();
    }

private:
    stack<int> stack1;
    stack<int> stack2;
};

 正确的：

class Solution
{
public:
    void push(int node) {
        stack1.push(node);
    }

    int pop() {
        int a ;
        if(stack2.empty() == true){
            while(stack1.empty() != true){
                a = stack1.top();
                stack2.push(a);
                stack1.pop();
            }
        }
        a =  stack2.top();
        stack2.pop();
        return a ;
    }

private:
    stack<int> stack1;
    stack<int> stack2;
};

return 这个要小心  ，return完了之后，下面的不会执行的了

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( self.s2 ==[]):
            while self.s1:
                a = self.s1[-1]
                self.s2.append(a)
                self.s1.pop()
        return self.s2.pop()