class MinStack {public:    /** initialize your data structure here. */    MinStack() {}        void push(int x) {        s1.push(x);        if (s2.empty() || x <= s2.top()) s2.push(x);    }        void pop() {        if (s1.top() == s2.top()) s2.pop();        s1.pop();    }        int top() {        return s1.top();    }        int getMin() {        return s2.top();    }    private:    stack
    
      s1, s2;};