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 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.

class Solution {
    public boolean isValidBST (TreeNode root) {
        if (root == null) {
            return true;
        }
        
        return validBST (root, null, null);
    }
    
    boolean validBST (TreeNode node, Integer min, Integer max) {
        if (node == null) {
            return true;
        }
        
        // Check if the current node's value is within the valid range
        if ((min != null && node.val <= min) || (max != null && node.val >= max)) {
            return false;
        }
        
        // Left Subtree Constraint: When you go to the left child, you update the max value 
        // because all nodes in the left subtree must be less than the current node's value. 
        // Hence, you check if the left child respects this constraint.

        // Right Subtree Constraint: When you go to the right child, you update the min value 
        // because all nodes in the right subtree must be greater than the current node's value. 
        // Hence, you check if the right child respects this constraint.

        // Recursively validate the left and right subtrees with updated min and max values
        return validBST(node.left, min, node.val) && validBST(node.right, node.val, max);

    }
}