1 public List<TreeNode> generateTrees(int n) {
2 List<TreeNode> res = new ArrayList<TreeNode>();
3 if(n <= 0) {
4 return res;
5 }
6 return helper(1, n);
7 }
8
9 private List<TreeNode> helper(int left, int right) {
10 List<TreeNode> res = new ArrayList<TreeNode>();
11 if(left > right) {
12 res.add(null);
13 return res;
14 }
15 for(int i = left; i <= right; i++) {
16 List<TreeNode> leftList = helper(left, i-1);
17 List<TreeNode> rightList = helper(i+1, right);
18 for(int j = 0; j < leftList.size(); j++) {
19 for(int k = 0; k < rightList.size(); k++) {
20 TreeNode root = new TreeNode(i);
21 root.left = leftList.get(j);
22 root.right = rightList.get(k);
23 res.add(root);
24 }
25 }
26 }
27 return res;
28 }
