Unique Binary Search Trees II

Given an integer 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. 1 3 3 2 1
2. \ / / / \ \
3. 3 2 1 1 3 2
4. / / \ \
5. 2 1 2 3

Solution:
1. /*
2. * Definition for a binary tree node.
3. * public class TreeNode {
4. * int val;
5. * TreeNode left;
6. * TreeNode right;
7. * TreeNode(int x) { val = x; }
8. * }
9. /
10. public class Solution {
11. public List generateTrees(int n) {
12. return helper(1, n);
13. }
14. 15. List helper(int lo, int hi) {
16. List res = new ArrayList<>();
17. 18. if (lo > hi) {
19. res.add(null);
20. return res;
21. }
22. 23. if (lo == hi) {
24. res.add(new TreeNode(lo));
25. return res;
26. }
27. 28. for (int i = lo; i <= hi; i++) {
29. List left = helper(lo, i - 1);
30. List right = helper(i + 1, hi);
31. 32. for (TreeNode l : left) {
33. for (TreeNode r : right) {
34. TreeNode root = new TreeNode(i);
35. root.left = l;
36. root.right = r;
37. res.add(root);
38. }
39. }
40. }
41. 42. return res;
43. }
44. }