#0239

Validate Binary Search Tree

MediumDSA28 min20 XP

Problem

A tree node is [value, left, right]. Return True if every node is strictly greater than all values in its left subtree and strictly less than all values in its right subtree.

Why This Matters

BST validation tests whether you understand that each node has a full allowed range, not only a parent comparison.

Function Signature

def is_valid_bst(root):

Examples

Example 1

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

OutputFalse

The value 4 is in the right subtree of 5, so it must be greater than 5, but it is not.

Constraints

Return the exact requested value.
Handle empty or small inputs when the prompt allows them.
Prefer the target DSA pattern over brute force when the input can grow.

CodePython

def is_valid_bst(root):
    """
    Input example: root = [5, [1, None, None], [4, [3, None, None], [6, None, None]]]
    Return: False
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return True

def is_valid_bst(root):
    """
    Input example: root = [5, [1, None, None], [4, [3, None, None], [6, None, None]]]
    Return: False
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return True

Visible browser tests run here when available.

Testcases2 visible / 3 hidden categories

Invalid deep value

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

ExpectedFalse

A local child check would miss the full ancestor range.

Valid tree

Input[8, [3, [1, None, None], [6, None, None]], [10, None, [14, None, None]]]

ExpectedTrue

Every subtree stays inside its allowed range.

Hidden Test Categories

duplicate values are invalidempty treeinvalid value several levels deep