Leetcode (Python): 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:

class Solution:
    # @return an integer
    def numTrees(self, n):
        return self.numTrees2(1, n)
        
    def numTrees2(self, minV, maxV):
        if minV >= maxV:
            return 1
        val = 0
        for i in range(minV,maxV+1):
            val = val + self.numTrees2(minV, i-1)*self.numTrees2(i+1, maxV)
        return val