Unique Binary Search Trees II

public class Solution {
    public ArrayList<TreeNode> generateTrees(int n) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        return generate(1, n);
    }
    public ArrayList<TreeNode> generate(int start, int end){
        ArrayList<TreeNode> subTree = new ArrayList<TreeNode>();
        if(start > end){
            subTree.add(null);
            return subTree;
        }
        
        for(int i = start; i <= end; i++){
            ArrayList<TreeNode> leftSubTree = generate(start, i - 1);
            ArrayList<TreeNode> rightSubTree = generate(i + 1, end);
            for(int j = 0; j < leftSubTree.size(); j++){
                for(int k = 0; k < rightSubTree.size(); k++){
                    TreeNode node = new TreeNode(i);
                    node.left = leftSubTree.get(j);
                    node.right = rightSubTree.get(k);
                    subTree.add(node);
                }
            }
        }
        return subTree;
    }
}