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63 lines
1.5 KiB
Python
63 lines
1.5 KiB
Python
"""
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98. Validate Binary Search Tree
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Difficulty: Medium
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https://leetcode.com/problems/validate-binary-search-tree/
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──────────────────────────────────────────────────
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Given the root of a binary tree, determine if it is a valid binary
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search tree (BST).
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A valid BST is defined as follows:
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• The left subtree of a node contains only nodes with keys strictly
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less than the node's key.
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• The right subtree of a node contains only nodes with keys strictly
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greater than the node's key.
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• Both the left and right subtrees must also be binary search trees.
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Example 1:
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Input: root = [2,1,3]
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Output: true
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Example 2:
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Input: root = [5,1,4,null,null,3,6]
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Output: false
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Explanation: The root node's value is 5 but its right child's value
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is 4.
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Constraints:
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• The number of nodes in the tree is in the range [1, 10^4].
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• -2^31 <= Node.val <= 2^31 - 1
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"""
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# Definition for a binary tree node.
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# class TreeNode:
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# def __init__(self, val=0, left=None, right=None):
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# self.val = val
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# self.left = left
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# self.right = right
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class Solution:
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def isValidBST(self, root: Optional[TreeNode]) -> bool:
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def valid(node, low, high):
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if not node:
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return True
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if not (low < node.val < high):
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return False
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return valid(node.left, low, node.val) and valid(node.right, node.val, high)
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return valid(root, float("-inf"), float("inf"))
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