95. 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
---

public class Solution {
    public int numTrees(int n) {
        
        
        if(n <= 1)  return 1;
        
        int sum=0;
        int left,right,root;
        
        for(root=1; root<=n; root++){
            left= numTrees(root-1);
            right = numTrees(n-root);
            sum += left * right;
        }
        
        return sum;
    }
    
}