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

Solution:

public class Solution {
    public int numTrees(int n) 
    {
        return numTrees(1, n);
    }
    
    public int numTrees(int min, int max) 
    {
        if(min>=max)
            return 1;
        int val = 0;
        for (int i=min; i<=max; i++)
        {
            val += numTrees(min, i-1) *numTrees(i+1, max);
        }
        return val;
    }
}