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

public class Solution {
	public List<TreeNode> generateTrees(int n) {

        return genTrees(1,n);
    }

    public List<TreeNode> genTrees (int start, int end)
    {

        List<TreeNode> list = new ArrayList<TreeNode>();

        if(start>end)
        {
            list.add(null);
            return list;
        }

        if(start == end){
            list.add(new TreeNode(start));
            return list;
        }

        List<TreeNode> left,right;
        for(int i=start;i<=end;i++)
        {

            left = genTrees(start, i-1);
            right = genTrees(i+1,end);

            for(TreeNode lnode: left)
            {
                for(TreeNode rnode: right)
                {
                    TreeNode root = new TreeNode(i);
                    root.left = lnode;
                    root.right = rnode;
                    list.add(root);
                }
            }

        }
        return list;
    }
}