LEETCODE: 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

递归之！

class Solution {public:    int numTrees(int n) {        if(n <= 1) return 1;        if(n == 2) return 2;        int result = 0;        for(int ii = 0; ii < n; ii ++) {            result += numTrees(ii) * numTrees(n - ii - 1);        }        return result;    }};