95. Unique Binary Search Trees I

将1到n的序列，选定任意一个数为根节点，然后分别求解左右字树的解。
同时递归求解左右子树。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def generateTrees(self, n: int) -> List[TreeNode]:
        if n <= 0:
            return []
        return self.helper(1, n)
    
    def helper(self, start, end):
        if start > end:
            return [None]
        result = []
        for i in range(start, end+1):
            left = self.helper(start, i-1)
            right = self.helper(i+1, end)
            for e in left:
                for f in right:
                    node = TreeNode(i)
                    node.left = e
                    node.right = f
                    result.append(node)
        return result