[LeetCode] Unique Binary Search Trees II

Thought:
Recursion.

Code:
public class Solution {
    public ArrayList<TreeNode> generateTrees(int n) {
        return generateBST(1,n);
    }
    public ArrayList<TreeNode> generateBST(int start, int end){
        ArrayList<TreeNode> result = new ArrayList<TreeNode>();
        if(start>end){
            result.add(null);
        }else if(start==end){
            result.add(new TreeNode(start));
        }else{
            for(int i=start;i<=end;i++){                
                ArrayList<TreeNode> left = generateBST(start,i-1); 
                ArrayList<TreeNode> right = generateBST(i+1,end);
                for(int k = 0;k<left.size();k++){
                    for(int j = 0;j<right.size();j++){ 
                        TreeNode root = new TreeNode(i);
                        root.left = left.get(k);
                        root.right = right.get(j);
                        result.add(root);
                    }
                }
            }
        }        
        return result; 
    }
}