不同的二叉搜索树 II

def solve(____):
    def generate(____):
        if l > r:
            return [None]
        trees = []
        for i in range(____):
            left = generate(____)
            right = generate(____)
            for lt in left:
                for rt in right:
                    root = TreeNode(____)
                    trees.append(____)
        return trees
    return generate(____, n) if n else []