Validate Binary Search Tree

Validate Binary Search Tree

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

Examples

Example 1:

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

    2
   / \
  1   3

Example 2:

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

    5
   / \
  1   4
     / \
    3   6

Explanation: The root's right child is 4, which is less than 5.
             Additionally, node 3 is in the right subtree of 5 but is less than 5.

Example 3:

Input: root = [5, 4, 6, null, null, 3, 7]
Output: false

    5
   / \
  4   6
     / \
    3   7

Explanation: Node 3 is in the right subtree of 5 but is less than 5.

Constraints

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

Expected Complexity

• Time: O(n)
• Space: O(h) where h is the height of the tree