95. Unique Binary Search Trees II

class Solution {
    public List<TreeNode> generateTrees(int n) {
        if(n==0)
            return new ArrayList<TreeNode>();
        return generateTrees(1, n);
    }
    private List<TreeNode> generateTrees(int i, int j) {
        List<TreeNode> res=new ArrayList<TreeNode>();
        if(i>j)
            res.add(null);
        else
        {
            for(int k=i;k<=j;k++)
            {
                List<TreeNode> left=generateTrees(i,k-1);
                List<TreeNode> right=generateTrees(k+1,j);
                for(TreeNode l:left)
                    for(TreeNode r:right)
                    {
                        TreeNode node=new TreeNode(k);
                        node.left=l;
                        node.right=r;
                        res.add(node);
                    }
            }
        }
        return res;
    }
}