Leetcode: Unique Binary Search Trees II

Given n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example,
Given n = 3, your program should return all 5 unique BST's shown below.

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

Solution:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; left = null; right = null; }
 * }
 */
public class Solution {
    public List<TreeNode> generateTrees(int n) {
        return generateTrees(1,n);
    }
    
    public List<TreeNode> generateTrees(int begin, int end) 
    {
        List<TreeNode> solution = new ArrayList<TreeNode>();
        if(begin>end)
        {
            solution.add(null);
            return solution;
        }
        for(int i=begin; i<=end; i++)
        {
            List<TreeNode> left = generateTrees(begin, i-1);
            List<TreeNode> right = generateTrees(i+1, end);
            for(TreeNode leftNode: left)
            {
                for (TreeNode rightNode: right)
                {
                    TreeNode node = new TreeNode(i);
                    node.left = leftNode;
                    node.right = rightNode;
                    solution.add(node);
                }
            }
            
        }
        return solution;
    }
}