Unique Binary Search Trees II

 1 public class Solution {
 2     public ArrayList<TreeNode> generateTrees(int n) {
 3         // IMPORTANT: Please reset any member data you declared, as
 4         // the same Solution instance will be reused for each test case.
 5         return generate(1, n);
 6     }
 7     public ArrayList<TreeNode> generate(int start, int end){
 8         ArrayList<TreeNode> subTree = new ArrayList<TreeNode>();
 9         if(start > end){
10             subTree.add(null);
11             return subTree;
12         }
13         
14         for(int i = start; i <= end; i++){
15             ArrayList<TreeNode> leftSubTree = generate(start, i - 1);
16             ArrayList<TreeNode> rightSubTree = generate(i + 1, end);
17             for(int j = 0; j < leftSubTree.size(); j++){
18                 for(int k = 0; k < rightSubTree.size(); k++){
19                     TreeNode node = new TreeNode(i);
20                     node.left = leftSubTree.get(j);
21                     node.right = rightSubTree.get(k);
22                     subTree.add(node);
23                 }
24             }
25         }
26         return subTree;
27     }
28 }