struct rbtree* successor(struct rbtree *node) {
    if (!node) return NULL;
    if (node->right) {
        /* 右子樹的最左子樹 */
        node = node->right;
        while (node->left) node = node->left;
        return node;
    } else {
        /* 向上直到當(dāng)前節(jié)點(diǎn)是其父節(jié)點(diǎn)的左子樹(后繼為父節(jié)點(diǎn))
           或者父節(jié)點(diǎn)為NULL(此時(shí)后繼節(jié)點(diǎn)不存在返回NULL) */
        struct rbtree *parent = _rb_parentof(node);
        while (parent && node == parent->right) {
            node = parent;
            parent = _rb_parentof(node);
        }
        return parent;
    }
}