自平衡二叉查找树，试过一些题，应该没什么bug。支持插入、删除、最大/最小值，求第k大，检索，前驱，后继；动态/静态内存都有，静态慢些，不过有些题用动态回MLE，切记-Self-balancing binary search tree, tried some of the questions, it should be no bug. Supports insert, delete, maximum/minimum, seeking the first k large, retrieval, precursor, successor　dynamic/static memory has static slower, but some problems with the dynamic back MLE, remember