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What is a heap / priority queue?

Q: What is a heap / priority queue?

**Intuition.** A heap is a tree that always keeps the smallest (or largest) element at the **root**, so you can peek the extreme in O(1) and pull it in O(log n). It is the engine behind a **priority queue** — a queue where you dequeue by priority, not arrival order. min-heap: every parent ≤ its children <line x1="100" y1="118" x2="140" y2="160" stroke="currentColor"

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Explain heaps.

heappriority-queue

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k largest with a heap

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import heapq

def k_largest(nums, k):
    heap = []                      # min-heap of size k
    for n in nums:
        heapq.heappush(heap, n)
        if len(heap) > k:
            heapq.heappop(heap)    # drop the smallest
    return sorted(heap, reverse=True)


# --- demo --- (or the built-in: heapq.nlargest(k, nums))
print(k_largest([3, 1, 5, 12, 2, 11], 3))   # [12, 11, 5]

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