/**
    knakul853
 */
class Solution {
public:
    bool isValidBST(TreeNode* root, long mn=LONG_MIN, long mx=LONG_MAX ) {
        if( !root ) return true;

         if( root->val <= mn || root->val >= mx) return 0;

        return isValidBST(root->left, mn, root->val ) && isValidBST(root->right, root->val, mx);

    }

};