Find Median From a Data Stream

Question

## Problem

Design a structure supporting addNum and findMedian over a stream.

## Example
```
addNum(1),addNum(2) -> 1.5; addNum(3) -> 2
```

## Constraints
- up to 5 × 10^4 calls

## Approach
Two heaps (max-heap lower half, min-heap upper half). Reported Asana question.