class Solution
{
public:
    void push(int n){s.push(n); }
    int pop(){int a;
        if(S.empty())for(;s.size();s.pop())
            S.push(s.top());
        a=S.top();S.pop();return a;
    }
private:
    stack<int>s,S;
};

给出一份极简解