Unique Binary Search Trees

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

没有做备忘录，否则效率更高。
本题相当于对于n个数，先拿出来一个数做根，剩下的数以各种方式分别给左子树和右子树。

class Solution {
public:
    int numTrees(int n) {
        int re =0;
        if(n == 1||n==0)return 1;
        for(int i = 0 ; i<n ;i++)
        re += numTrees(i)*numTrees(n-i-1);
        return re;
    }
};