/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 /*
 Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a 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.
Example 1:
    2
   / \
  1   3
Binary tree [2,1,3], return true.
Example 2:
    1
   / \
  2   3
Binary tree [1,2,3], return false.
*/

class Solution {
public:
    TreeNode* prev = NULL;
    bool isValidBST(TreeNode* root) 
    {
        if (root)
        {
            if (!isValidBST(root->left))
                return false;
            if (prev && root->val <= prev->val)
                return false;
            prev = root; cout<<prev->val<<endl;
            return isValidBST(root->right);
        }
 
    return true;    
    }
};

Update Check BST or not
 Update Check BST or not
