Unique Binary Search Trees

Example:

Input: 3
Output: 5
Explanation:
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

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

        int sum=0;
        for(int i=1;i<=n;++i)//每个结点做一次根
            sum+=numTrees(i-1)*numTrees(n-i);
        return sum;
    }
};