说明：  最优二叉搜索树问题具有最优子结构性质，即其任意一棵子树都是一棵独立的最优二叉搜索树，所以可以采用动态规划方法，并且在每次更新时保存子树的根。
(Optimal binary search tree problem has optimal substructure property that any of its sub-tree are an independent optimal binary search tree, so you can use dynamic programming methods, and save each update subtree roots.)