LeetCode-95 Unique Binary Search Trees II

public List<TreeNode> generateTrees(int n) {
        if(n < 1) {
            List<TreeNode> list = new ArrayList<TreeNode>();
            list.add(null);
            return list;
        }
        return generateTree(1, n);
    }
    
    public List<TreeNode> generateTree(int start ,int over) {
        List<TreeNode> list = new ArrayList<TreeNode>();
        if(start > over) {
            list.add(null);
            return list;
        }
        if(start == over) {
            list.add(new TreeNode(start));
            return list;
        }
        
        for(int i=start; i<=over; i++) {
            List<TreeNode> leftList = generateTree(start ,i-1);
            List<TreeNode> rightList = generateTree(i+1 ,over);
            for(TreeNode left : leftList) {
                for(TreeNode right : rightList) {
                    TreeNode root = new TreeNode(i);
                    root.left = left;
                    root.right = right;
                    list.add(root);
                }
            }
        }
        return list;
    }