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Validate Binary Search Tree

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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.

Test Cases

Copy an input into the main harness and Run to verify
Input
root = [2,1,3]
Expected Output
true
Input
root = [5,1,4,null,null,3,6]
Expected 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, 10^4].
  • -2^31 <= Node.val <= 2^31 - 1

Hints

Hint 1 — click to reveal

Comparing only parent-child is not enough — subtrees must respect ALL ancestors.

Hint 2 — click to reveal

Pass down a valid (min, max) range to each node.

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