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

题目地址： https://oj.leetcode.com/problems/unique-binary-search-trees/

这道题用递归的解法，很简单...

代码：

class Solution {
public:
    int numTrees(int n) {
        if(n==0||n==1) return 1;

		int num=0;
		for(int i=1;i<=n;i++){
			//返回左边子树的个数和右边子树的组合数
			num+=numTrees(i-1)*numTrees(n-i);
		}
		return num;
    }
};