95. Unique Binary Search Trees I

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def generateTrees(self, n):
        """
        :type n: int
        :rtype: List[TreeNode]
        """
        def node(val,left,right):
            node=TreeNode(val)
            node.left=left
            node.right=right
            return node
        def tree(first,last):
            return [node(root,left,right) for root in range(first,last+1) for left in tree(first,root-1) for right in tree(root+1,last)] or [None]
        
        
        return [] if n==0 else tree(1,n)