Find Median from Data Stream

The **MedianFinder** class supports finding the median of a stream of numbers. Implement the following: - `MedianFinder()` initializes the object. - `void addNum(int num)` adds the integer `num` from the data stream to the data structure. - `double findMedian()` returns the median of all elements so far. **Example:** ``` Input: ["MedianFinder","addNum","addNum","findMedian","addNum","findMedian"] [[],[1],[2],[],[3],[]] Output: [null,null,null,1.5,null,2.0] ``` If the size of the list is even, there is no middle value, and the median is the mean of the two middle values.