使用红黑树编写“背单词”应用，根据熟悉度进行排序，熟悉度相同根据字典序排序。可添加，删除单词，也可以修改单词的熟悉度。可查找第k个单词，或计算kth到lth的单词熟悉度综合。-Use red-black tree to write " back the word" application, based on familiarity sort of familiarity same lexicographically sorted. You can add, delete wor