//Iterative inorder traversal solution:

 bool isValidBST(TreeNode* root) {
        stack<TreeNode*> to_process;
        TreeNode* curr = root;
        TreeNode* min_sofar = nullptr;

        while(curr || !to_process.empty()) {
            while (curr) {
                to_process.push(curr);
                curr = curr->left;
            }

            curr = to_process.top();
            to_process.pop();

            if (min_sofar && min_sofar->val >= curr->val) return false;

            min_sofar = curr;
            curr = curr->right;
        }

        return true;
    }