Is Validate Binary Search Tree easy or hard?

Validate Binary Search Tree is considered a Medium difficulty problem on LeetCode. The challenge comes from correctly applying BST properties across the entire subtree rather than only comparing immediate parent and child nodes.

Validate Binary Search Tree Python/Java solution

In Python or Java, the standard solution performs an in-order traversal and tracks the previous node value. If the current node value is less than or equal to the previous value, return false. Otherwise continue traversal until all nodes are processed.

How to solve Validate Binary Search Tree in O(n)?

Perform an in-order traversal and maintain a variable storing the previously visited node value. For each node, check that the current value is strictly greater than the previous one. If the order ever breaks, the tree is not a valid BST. This ensures O(n) time since every node is processed once.

What is the best approach for Validate Binary Search Tree?

The most common approach uses in-order traversal of the tree. A valid binary search tree produces a strictly increasing sequence when traversed in-order. By tracking the previously visited value and ensuring each new value is greater, you can validate the BST in O(n) time with O(h) space.

Is Validate Binary Search Tree asked at Google/Amazon/Meta?

Validate Binary Search Tree is a common interview question at major tech companies including Google, Amazon, and Meta. It tests understanding of tree traversal, recursion, and the core properties of binary search trees.

What data structure is used in Validate Binary Search Tree?

The problem uses a binary tree structure and typically relies on depth-first search traversal. Implementations use recursion or an explicit stack to perform in-order traversal while checking BST ordering constraints.

What is the time complexity of Validate Binary Search Tree?

The optimal solution runs in O(n) time because each node in the tree is visited exactly once during traversal. Space complexity is O(h), where h is the height of the tree, due to recursion stack or an explicit stack used in iterative traversal.

Description Solution Video 9:56

98. Validate Binary Search Tree

Medium36.1% acceptance18 companies

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 less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.

Example 1:

Input: root = [2,1,3]
Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

Constraints:

The number of nodes in the tree is in the range [1, 10⁴].
-2³¹ <= Node.val <= 2³¹ - 1

Asked at 18 companies

Wix Yahoo Ola Millennium Bloomberg Expedia Amazon Microsoft Meta Oracle

IBM Citadel

+6 more

Google Apple Salesforce Goldman Sachs LinkedIn Yandex

Topics

9:56

Still stuck?Read 3 approaches, or watch a walkthrough.

Open solution

98. Validate Binary Search Tree

Asked at 18 companies

Topics

Similar problems