辅助栈关键在于每次都有入栈，入栈元素为最小元素，个数与s保持一致

private:
    stack<int> s,minStack;
public:
    void push(int value) {
        if(minStack.empty()){
            minStack.push(value);
        }
        s.push(value);
        if(minStack.empty()){
            minStack.push(value);
        }
        else{
            if(minStack.top()>value)
                minStack.push(value);
            else
                minStack.push(minStack.top());//每次让当前栈最小值入栈
        }
    }
    void pop() {
        s.pop();
        minStack.pop();
    }
    int top() {
        return s.top();
    }
    int min() {
        return minStack.top();
    }
};