Heap / Priority Queue

heapq is a min-heap over a plain list. Great for top-k and streaming.

Core syntax

• Push / pop — heapq.heappush(h, x) / heapq.heappop(h).
• Heapify — heapq.heapify(nums) (O(n), in place).
• Max-heap — push -x, negate on pop.

import heapq
h = []
for x in nums:
    heapq.heappush(h, x)
    if len(h) > k:
        heapq.heappop(h)    # keep k largest

Watch out

• Tuples sort by first element: (priority, item).