思路：
    两个优先级队列维持，一个容纳较小的数字，一个容纳较大的数字

    1.优先级队列的队头在容器尾部
    image.png

    https://blog.csdn.net/weixin_36888577/article/details/79937886

    2.只要有一个操作数是double，商就是double

    1.     priority_queue<int> big;//大顶堆，默认
    2.     priority_queue<int, vector<int>, greater<int> > small;//小顶堆 
    3.     void Insert(int num)
    4.     {
    5.         if(big.empty() || num <= big.top())big.push(num);
    6.         else small.push(num);
    7.         if(big.size() > small.size() + 1){
    8.             small.push(big.top());
    9.             big.pop();
    10.         }
    11.         if(small.size() > big.size()){
    12.             big.push(small.top());
    13.             small.pop();
    14.         }
    15.     }
    17.     double GetMedian()
    18.     { 
    19.         int len = small.size() + big.size();
    20.         if(len % 2 == 1)return big.top();
    21.         else return (small.top() + big.top()) / 2.0;
    22.     }