Solved Arai60/98. Validate Binary Search Tree #28
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oda
reviewed
Jun 10, 2025
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| - 初めに思い付いたのは左右との比較を行い、どんどん下りていく方法 | ||
| - ただ、直後のノードを確かめるだけでは不十分なので毎回深掘る実装をする | ||
| - 時間計算量は最大O(n^2)になってしまうので流石に修正する |
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線形時間にする方法として「ペアを返す再帰にすると線形時間になる」を使う方法もあります。
再帰が (最小、最大、is_valid) の3つ組を返すようにするということです。
https://docs.google.com/document/d/11HV35ADPo9QxJOpJQ24FcZvtvioli770WWdZZDaLOfg/edit?tab=t.0#heading=h.r1pr4pvhjsjx
Fuminiton
reviewed
Jun 11, 2025
| MAX_VAL = 2**31 | ||
| class Solution: | ||
| def isValidBST(self, root: Optional[TreeNode]) -> bool: | ||
| nodes_and_ranges: list[tuple[int]] = [(root, MIN_VAL, MAX_VAL)] |
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MIN_VAL、MAX_VALを定義せず、-sys.maxint - 1、sys.maxintを使って初期化しても、意図は伝わるかと思いました。
| return False | ||
| nodes_and_ranges.append((node.left, min_val, node.val)) | ||
| nodes_and_ranges.append((node.right, node.val, max_val)) | ||
| return True |
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とても読みやすかったです。
Generatorを使った実装は、inorder順がソート順になるという特徴を利用していて直感的なので、練習してみると良いと思います。
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問題文：https://leetcode.com/problems/validate-binary-search-tree/description/