find median from data stream

import heapq

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

    def addNum(self, num: int) -> None:
        # push to maxheap (as negative to simulate max-heap)
        heapq.heappush(self.maxheap, -num)

        # ensure order: every item in maxheap <= every item in minheap
        heapq.heappush(self.minheap, -heapq.heappop(self.maxheap))

        # balance sizes: maxheap can have one extra
        if len(self.minheap) > len(self.maxheap):
            heapq.heappush(self.maxheap, -heapq.heappop(self.minheap))

    def findMedian(self) -> float:
        if len(self.maxheap) > len(self.minheap):
            return float(-self.maxheap[0])
        return (-self.maxheap[0] + self.minheap[0]) / 2.0