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

//卡特兰数

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

	    int buf[n+1];
	    buf[0] = 1;
	    buf[1] = 1;

	    for (int i = 2; i <= n; i++)
	    {
		    int result = 0;
		    for (int j = 0; j < i; j++)
		    {
			    result += buf[j] * buf[i-1-j];
		    }

		    buf[i] = result;
	    }

	    return buf[n];
    }
};