用两个栈实现一个队列

class Solution
{
public:
    void push(int node) {

        stack1.push(node);
    }

    int pop() {       
        int a;
        if(stack2.empty()) //需要判断栈2是否为空
        {
            while(!stack1.empty())
            {              
                stack2.push(stack1.top());
                stack1.pop();
            }           

        }
         a=stack2.top();       
        stack2.pop();
        return a;
    }
private:
    stack<int> stack1;
    stack<int> stack2;
};