Create Unique Binary Search Trees 

递归，划分成子问题，假设左右子树的情况为A,B 则共有A*B中情况。如果划分足够的细致，划分到只有一个就为1种情况，两个就是两种情况。

Given n, how many structurally unique BST's (binary search trees) that store values 1...n?

For example,
Given n = 3, there are a total of 5 unique BST's.

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3