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

 1 class Solution
 2 {
 3 public:
 4     void push(int node) {
 5         stack1.push(node);
 6     }
 7 
 8     int pop() {
 9         int a ;
10         if(stack2.empty() == true){
11             while(stack1.empty() != true){
12                 a = stack1.top();
13                 stack2.push(a);
14                 stack1.pop();
15             }
16         }
17         return stack2.top();
18         stack2.pop();
19     }
20 
21 private:
22     stack<int> stack1;
23     stack<int> stack2;
24 };

正确的：

 1 class Solution
 2 {
 3 public:
 4     void push(int node) {
 5         stack1.push(node);
 6     }
 7 
 8     int pop() {
 9         int a ;
10         if(stack2.empty() == true){
11             while(stack1.empty() != true){
12                 a = stack1.top();
13                 stack2.push(a);
14                 stack1.pop();
15             }
16         }
17         a =  stack2.top();
18         stack2.pop();
19         return a ;
20     }
21 
22 private:
23     stack<int> stack1;
24     stack<int> stack2;
25 };

return 这个要小心，return完了之后，下面的不会执行的了

python：

 1 # -*- coding:utf-8 -*-
 2 class Solution:
 3     def __init__(self):
 4         self.s1 = []
 5         self.s2 = []
 6     def push(self, node):
 7         # write code here
 8         self.s1.append(node);
 9     def pop(self):
10         # return xx
11         if( self.s2 ==[]):
12             while self.s1:
13                 a = self.s1[-1]
14                 self.s2.append(a)
15                 self.s1.pop()
16         return self.s2.pop()