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

果然是大半夜了，脑子不好使了，这么简单的题居然还改了半天。

class Solution {
public:
int numTrees(int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if(n == 0) return 1;
if(n == 1) return 1;
int total = 0;
for(int i = 0; i< n; ++i){
int left = numTrees(i);
int right = numTrees(n-i - 1);
total += left * right;
}
return total;
}
};