[二叉搜索树]不同的二叉搜索树（给出具体的树）

# 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[Optional[TreeNode]]:

        def buildtree(start,end):

            if start > end:
                return [None]

            alltrees = []

            for i in range(start,end+1):
                
                left = buildtree(start,i-1)
                right = buildtree(i+1,end)
                
                for left_tree in left:
                    for right_tree in right:
                        root = TreeNode(i)
                        root.left = left_tree
                        root.right = right_tree
                        alltrees.append(root)
            
            return alltrees

        return buildtree(1,n)