Enumerate binary trees - EPI

Enumerate binary trees  UniqueBinaryTreesAll.java

  public static List<BinaryTree<Integer>> generateAllBinaryTrees(int n) {
    return generateAllBinaryTreesHelper(1, n);
  }

  private static List<BinaryTree<Integer>> generateAllBinaryTreesHelper(
      int start, int end) {
    List<BinaryTree<Integer>> result = new ArrayList<>();
    if (start > end) {
      result.add(null);
      return result;
    }

    for (int i = start; i <= end; ++i) {
      // Tries all possible combinations of left subtrees and right subtrees.
      List<BinaryTree<Integer>> leftresult = generateAllBinaryTreesHelper(start,
          i - 1);
      List<BinaryTree<Integer>> rightresult = generateAllBinaryTreesHelper(
          i + 1, end);
      for (BinaryTree<Integer> left : leftresult) {
        for (BinaryTree<Integer> right : rightresult) {
          result.add(new BinaryTree<>(i, left, right));
        }
      }
    }
    return result;
  }