/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    
    bool isValidBST(TreeNode* root) {
       
        return check(root, LONG_MIN, LONG_MAX);
    }
    
   bool check(TreeNode* root, long minval, long maxval)
   {
       if(!root) return true;
       if(root->val>=maxval||root->val<=minval) return false;
       
       return check(root->left, minval, root->val)&&check(root->right, root->val, maxval);
   }
};