95 unique binary search trees ii

def (self, n):
    
    if n == 0:
        return []
    return self.dfs(1, n)
    
def dfs(self, start, end):
    if start > end: return [None]
    res = []
    for rootval in range(start, end+1):
        LeftTree = self.dfs(start, rootval-1)
        RightTree = self.dfs(rootval+1, end)
        for i in LeftTree:
            for j in RightTree:
                root = TreeNode(rootval)
                root.left = i
                root.right = j
                res.append(root)
    return res