BSTnode* remove_node(BSTnode* root, int value) {
    // When root is null
    if (root == NULL) {
        return NULL;
    }

    // If value lies in left subtree
    if (value < root->val) {
        root->left = remove_node(root->left, value);
        return root;
    }
    // If value lies in right subtree
    else if (value > root->val) {
        root->right = remove_node(root->right, value);
        return root;
    }

    // if value is located in root node
    else {

        // node with only one child or no child
        if(root->left == NULL && root->right == NULL) {
            delete root;
            return NULL;
        } else if (root->left == NULL) {
            BSTnode *son = root->right;
            son->parent = root->parent;
            delete root;
            return son;

        } else if (root->right == NULL) {
            BSTnode *son = root->left;
            son->parent = root->parent;
            delete root;
            return son;
        }

        // node with two children: The inorder successor is the new value
        BSTnode* temp = min(root->right);

        // Copy the inorder successor's content to this node
        root->val = temp->val;

        // Delete the inorder successor
        root->right = remove_node(root->right, temp->val);
    }
    return root;
}