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

 ***/

class Solution {

public:

       int getnumTrees(int st, int ed) {

              if(st >= ed) return 1;

              int sum = 0;

              for(int i=st; i<=ed; i++) {

                     sum += getnumTrees(st, i-1) * getnumTrees(i+1, ed);

              }

              return sum;

       }

       int numTrees(int n) {

              // Start typing your C/C++ solution below

              // DO NOT write int main() function

              return getnumTrees(1, n);

       }

};