Unique Binary Search Trees II

 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<TreeNode> generateTrees(int n) {
12         if (n == 0) {
13             return new ArrayList<TreeNode>();
14         }
15         return generate(1, n);
16     }
17     
18     private List<TreeNode> generate(int start, int end) {
19         List<TreeNode> result = new ArrayList<>();
20         if (start > end) {
21             result.add(null);
22             return result;
23         }
24         for (int i = start; i <= end; i++) {
25             List<TreeNode> left = generate(start, i - 1);
26             List<TreeNode> right = generate(i + 1, end);
27             for (TreeNode l : left) {
28                 for (TreeNode r : right) {
29                     TreeNode root = new TreeNode(i);
30                     root.left = l;
31                     root.right = r;
32                     result.add(root);
33                 }
34             }
35         }
36         return result;
37     }
38 }