===== ISTEM =====
Solve the following programming task.

TASK: Median Finder on a Data Stream

Write a 'MedianFinder' class that accepts numbers from a data stream and computes the running median with O(log n) insertion and O(1) lookup. It must have 'add_num(num)' and 'find_median()' methods.

STARTER CODE (python):
class MedianFinder:
    def __init__(self):
        pass
    def add_num(self, num: int) -> None:
        pass
    def find_median(self) -> float:
        pass


RULES:
- Keep the function name and signature EXACTLY as given.
- Return working code only. No explanations.
- Put the code in a single ``` block.
- Try to solve it first. If you genuinely cannot, write only this single
  line instead of producing faulty code: CANNOT_SOLVE

===== HAM YANIT =====
```python
import heapq

class MedianFinder:
    def __init__(self):
        self.left = []   # max-heap (store negatives)
        self.right = []  # min-heap

    def add_num(self, num: int) -> None:
        if not self.left or num <= -self.left[0]:
            heapq.heappush(self.left, -num)
        else:
            heapq.heappush(self.right, num)

        # Rebalance heaps so that len(left) == len(right) or len(left) == len(right)+1
        if len(self.left) > len(self.right) + 1:
            heapq.heappush(self.right, -heapq.heappop(self.left))
        elif len(self.right) > len(self.left):
            heapq.heappush(self.left, -heapq.heappop(self.right))

    def find_median(self) -> float:
        if len(self.left) > len(self.right):
            return float(-self.left[0])
        return (-self.left[0] + self.right[0]) / 2.0
```