Example 1: What is the output for input root = [2,1,3]?

Output: true. Node 1 root 2 (valid right child). Both subtrees are valid BSTs.

MediumTrees

Validate Binary Search Tree

Q: Example 2: What is the output for input root = [5,1,4,null,null,3,6]?

Output: false. The right child of the root has value 4, which is less than the root value 5. This violates the BST property.

amazongooglemicrosoftbloombergapple

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:

  2
 / \
1   3

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

Example 2:

    5
   / \
  1   4
     / \
    3   6

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.

Examples

Example 1

Input: root = [2,1,3]

Output: true

Explanation: Node 1 < root 2 (valid left child). Node 3 > root 2 (valid right child). Both subtrees are valid BSTs.

Example 2

Input: root = [5,1,4,null,null,3,6]

Output: false

Explanation: The right child of the root has value 4, which is less than the root value 5. This violates the BST property.

Constraints

-The number of nodes in the tree is in the range [1, 10^4].
--2^31 <= Node.val <= 2^31 - 1

Optimal Complexity

Time

O(n)

Space

O(h)

Practice this problem with an AI interviewer

TechInView conducts a full voice mock interview — the AI asks clarifying questions, evaluates your approach, watches you code, and scores you on 5 dimensions. Just like a real FAANG interview.

Start a free interview