[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

很简单。

class Solution {
public:
    int numTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int * a;
        a = new int[n+1];
        a[0] = 1;
        for(int i = 1;i <= n;i++)
        {
            a[i] = 0;
            for(int j = 0;j<=i-1;j++)
                a[i] += a[j] * a[i - 1 - j]; 
        }
        return a[n];
    }
};