95. Unique Binary Search Trees II

 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     }