leetcode 95 不同的二叉搜索树 ii

# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class (object):
    def generateTrees(self, n):
        """
        :type n: int
        :rtype: List[TreeNode]
        """
        
        def helper(l):
            if not l:
                return [None]
            elif len(l) == 1:
                return [TreeNode(l[0])]
            ans = []
            for i, n in enumerate(l):
                for left in helper(l[:i]):
                    for right in helper(l[i+1:]):
                        root = TreeNode(n)
                        if left is not None:
                            root.left = left
                        if right is not None:
                            root.right = right
                        ans.append(root)
            return ans
        
        if n <= 0:
            return []
        
        return helper([i for i in range(1, n+1)])