Determine if a given binary tree is a binary search tree (BST).

Our solution to validate whether a binary tree is a Binary Search Tree (BST) uses a recursive approach. We traverse the tree while maintaining a range for valid node values (min and max). At each node, we check whether its value lies within the given range:

• If the node’s value is not within the allowed range, we return False, indicating the tree is not a valid BST.
• If the node’s value is valid, we recursively check its left and right subtrees:
  • The left subtree must have values strictly less than the current node's value.
  • The right subtree must have values strictly greater than the current node's value.

For the left subtree, the range becomes (min, current_node_value). For the right subtree, the range becomes (current_node_value, max).

We use nullptr to indicate unbounded ranges when there is no minimum or maximum restriction (e.g., for the root node or outermost nodes).

If all nodes satisfy their respective conditions, we return True, confirming that the tree is a valid BST.

Time Complexity: The solution has a time complexity of O(n), where n is the number of nodes in the binary tree. This is because we traverse each node exactly once, performing constant-time checks at each node.

Space Complexity: The space complexity is O(h), where h is the height of the tree. This space is used by the recursion stack. In the worst case (for an unbalanced tree), the height could be n, making the space complexity O(n). For a balanced tree, the height would be O(log n).