堆積 (Heap) 是一種特殊的完全二元樹，能在 O(1) 時間取得最大/最小值，O(log n) 時間插入和刪除。優先佇列 (Priority Queue) 基於堆積實作。
Notes:
- 看到「第 K 大/小」、「Top K」等關鍵字，優先考慮 Heap
- Kotlin/Java 中用 PriorityQueue 實作，預設為 Min Heap
- 兩個 Heap（一大一小）可以高效維護中位數
跨倉庫導讀#
- 對應理論章節：樹 ↗（堆/優先佇列）
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