What is the optimal time complexity of solving Validate BST?

The optimized solution achieves a time complexity of O(n) by using sorting, mathematical shortcuts, or hash table lookups.

What is the auxiliary space complexity for Validate BST?

The space complexity is O(1) since we only use a minimal/constant amount of extra memory for tracking variables (or auxiliary space if dynamic structures are allocated).

What are the typical edge cases we must handle in Validate BST?

Key edge cases include empty collections, negative or large bounds causing integer overflow, arrays with only one element, or inputs where target criteria are not met.

How can we optimize the brute force approach for Validate BST?

Brute force methods often inspect all possible combinations. We optimize this by sorting the input list to enable binary search, or tracking processed items in a set to avoid redundant nested loop lookups.

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Validate BST

Learn how to solve the 'Validate BST' problem. This detailed resource details brute force and optimized approaches.

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Problem Statement

Easy

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid BST is defined as follows:

- The left subtree of a node contains only nodes with keys strictly less than the node's key.

- The right subtree of a node contains only nodes with keys strictly greater than the node's key.

- Both the left and right subtrees must also be binary search trees.

The tree is represented as a level-order list. Implement a function isValidBST(root: list) -> bool.

Constraints

•The number of nodes in the tree is in the range [1, 10000]
•-2^31 <= Node.val <= 2^31 - 1

Examples

Example 1

Input

[2,1,3]

Output

True

Explanation

The left child 1 < root 2, and right child 3 > root 2. Valid BST.

Example 2

Input

[5,1,4,None,None,3,6]

Output

False

Explanation

The right child of root is 4, which is less than 5. Also, node 3 is in the right subtree of 5 but is less than 5. Not a valid BST.

Example 3

Input

[5,4,6,None,None,3,7]

Output

False

Explanation

Node 3 is in the right subtree of 5 but has value 3 < 5. Not valid.

Need a Hint?

Perform a recursive tree traversal (DFS) or level-order traversal (BFS) using a queue/stack.

Edge Cases to Watch

Empty list or null input variables
Single item lists/arrays
Extremely large input bounds causing integer or stack overflow

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