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

Java

public class Solution {
    public List<TreeNode> generateTrees(int n) {
        return generateTrees(1, n);
    }

    private List<TreeNode> generateTrees(int start, int end) {
        List<TreeNode> rst = new LinkedList<TreeNode>();
        if(start > end) {
            rst.add(null);
            return rst;
        }

        for(int i = start; i <= end; i++) {
            List<TreeNode> lefts = generateTrees(start, i - 1);
            List<TreeNode> rights = generateTrees(i + 1, end);
            for(TreeNode left : lefts) {
                for(TreeNode right : rights) {
                    TreeNode root = new TreeNode(i);
                    root.left = left;
                    root.right = right;
                    rst.add(root);
                }
            }
        }
        return rst;
    }
}