用双栈实现队列

class Solution
{
public:
    void push(int node) {
        s1.push(node);
    }

    int pop() {
        if(s2.empty()){
            while(!s1.empty()){
                s2.push(s1.top());
                s1.pop();
            }
        }
        if(s2.empty())
            return -1;
        int t = s2.top();
        s2.pop();
        return t;
    }

private:
    stack<int> s1;
    stack<int> s2;
};