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

原题链接：https://oj.leetcode.com/problems/unique-binary-search-trees/

题目：给定n,有多少个1到n的二叉搜索树？

思路：问题可以分成两步，一是(1,i-1)的个数，一是(i+1,n)的个数，两者相乘，就是总的个数。

public int numTrees(int n) {if(n == 1 || n == 0)return 1;else{int num = 0;for(int i=1;i<=n;i++)num += numTrees(i - 1) * numTrees(n - i);return num;}}