The median is the middle value in a sorted list of integers. For lists of even length, there is no middle value, so the median is the mean of the two middle values.

For example:

• For arr = [1,2,3], the median is 2.
• For arr = [1,2], the median is (1 + 2) / 2 = 1.5

Implement the MedianFinder class:

• MedianFinder() initializes the MedianFinder 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 1:

Input:
["MedianFinder", "addNum", "1", "findMedian", "addNum", "3" "findMedian", "addNum", "2", "findMedian"]

Output:
[null, null, 1.0, null, 2.0, null, 2.0]

Explanation:
MedianFinder medianFinder = new MedianFinder();
medianFinder.addNum(1);    // arr = [1]
medianFinder.findMedian(); // return 1.0
medianFinder.addNum(3);    // arr = [1, 3]
medianFinder.findMedian(); // return 2.0
medianFinder.addNum(2);    // arr[1, 2, 3]
medianFinder.findMedian(); // return 2.0

Constraints:

• -100,000 <= num <= 100,000
• At most 50,000 calls will be made to addNum and findMedian.
• findMedian will only be called after adding at least one integer to the data structure.