// returns -1 if it is not bst, otherwise it is bst
int ifBST(BSTnode * root){
    odp o;
    o.ok=true;
    if(root == NULL){
        return 0;
    }
    else{
        int leftHeight = ifBST(root->left) , rightHeight = ifBST(root->right);
        if(leftHeight == -1 || rightHeight == -1 || abs(rightHeight - leftHeight)>1){
            return -1;
        }
        return  1 + max(leftHeight, rightHeight);
    }
}