递归
考点：
递归思想
# 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 []
def digui(start, end):
if start > end:
return [None]
result = []
for i in range(start, end+1):
lefts = digui(start, i-1)
rights = digui(i+1, end)
for j in lefts:
for k in rights:
cur = TreeNode(i)
cur.left = j
cur.right = k
result.append(cur)
return result
return digui(1, n)
