<O(n),O(1)>的LCA

https://www.topcoder.com/community/competitive-programming/tutorials/range-minimum-query-and-lowest-common-ancestor/
https://www.topcoder.com/community/data-science/data-science-tutorials/range-minimum-query-and-lowest-common-ancestor/#A O(N), O(1) algorithm for the restricted RMQ