[LeetCode] Unique Binary Search Trees

Problem:

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:O(n)

public class Solution {
    public int numTrees(int n) {
        if(n==0)
            return 1;
        if(n == 1)
            return 1;
        if(n == 2)
            return 2;
        int ret = 0;
        for(int k = 1; k<=n ;k++)
            ret+= (numTrees(k-1)* numTrees(n-k));
            
        return ret;
    }
}