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;
}
};