Find Median from Data Stream

from heapq import heappush, heappop

class MedianFinder:
    def __init__(self):
        self.small = []  # max heap (negative values)
        self.large = []  # min heap
    
    def addNum(self, num):
        if not self.small or num <= -self.small[0]:
            heappush(self.small, -num)
        else:
            heappush(self.large, num)
        
        # Balance heaps
        if len(self.small) > len(self.large) + 1:
            heappush(self.large, -heappop(self.small))
        elif len(self.large) > len(self.small):
            heappush(self.small, -heappop(self.large))
    
    def findMedian(self):
        if len(self.small) > len(self.large):
            return float(-self.small[0])
        return (-self.small[0] + self.large[0]) / 2