Unique Binary Search Trees II

public class Solution {
    public List<TreeNode> generateTrees(int n) {
        int[] num = new int[n];
        for(int i = 0; i < n; i++)
        {
            num[i] = i + 1;
        }
        return findNode(num, 0, n);
    }
    public List<TreeNode> findNode(int[] num, int start, int end)
    {
        if(start == end)
        {
            List<TreeNode> list = new ArrayList<TreeNode>();
            list.add(null);
            return list;
        }
        if(start + 1 == end) 
        {
            List<TreeNode> list = new ArrayList<TreeNode>();
            list.add(new TreeNode(num[start]));
            return list;
        }
        List<TreeNode> rst = new ArrayList<TreeNode>();
        for(int i = start; i < end; i++)
        {
           
            List<TreeNode> left = findNode(num, start, i);
            List<TreeNode> right = findNode(num, i + 1, end);
            for(TreeNode l: left)
            {
                for(TreeNode r: right)
                {
                    TreeNode root = new TreeNode(num[i]);
                    root.left = l;
                    root.right = r;
                    rst.add(root);
                }
            }
        }
        return rst;
    }
}