148Implement a median tracker for a real-time data stream

A monitoring dashboard receives response times one by one and must show the median latency at any moment (the median resists outliers better than the average). Implement class MedianFinder (LeetCode 295):

• addNum(num): add a number from the stream.
• findMedian(): the median of all numbers so far: the middle value for an odd count, the average of the two middle values for an even count. Return null before any number was added.
• size: how many numbers were added.

addNum must be O(log n) and findMedian O(1), so it keeps up with 100,000 events. JavaScript has no built-in heap: implement one.

const finder = new MedianFinder();
finder.addNum(1);
finder.addNum(2);
finder.findMedian(); // 1.5
finder.addNum(3);
finder.findMedian(); // 2