Medium · Trees

Is BST

Given the root of a binary tree, return true if it is a valid binary search tree (every node's value is strictly greater than all values in its left subtree and strictly less than all values in its right subtree) and false otherwise. An empty tree is valid.

{
  "tree": [5, 3, 8, 2, 4, 7, 9]
}

{
  "tree": [5, 3, 8, 1, 6]
}