UniqueBinary

class Solution {
    public List<TreeNode> generateTrees(int n) {
        return n > 0 ? generate_trees(1, n) : new ArrayList<>();
    }

    private List<TreeNode> generate_trees(int start, int end) {
        List<TreeNode> all_trees = new ArrayList<>();
        if (start > end) {
            all_trees.add(null);
            return all_trees;
        }

        for (int i = start; i <= end; i++) {
            List<TreeNode> left_trees = generate_trees(start, i - 1);
            List<TreeNode> right_trees = generate_trees(i + 1, end);

            for (TreeNode l : left_trees) {
                for (TreeNode r : right_trees) {
                    TreeNode current_tree = new TreeNode(i);
                    current_tree.left = l;
                    current_tree.right = r;
                    all_trees.add(current_tree);
                }
            }
        }
        return all_trees;
    }
}