leetcode-95-不同的二叉搜索树②*

题目描述：

方法一：递归

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

class Solution:
    def generateTrees(self, n: int) -> List[TreeNode]:
        def generate_trees(start,end):
            if start > end:
                return [None,]
            all_trees = []
            for i in range(start,end+1):
                left_trees = generate_trees(start,i-1)
                right_trees = generate_trees(i+1,end)
                for l in left_trees:
                    for r in right_trees:
                        current_tree = TreeNode(i)
                        current_tree.left = l
                        current_tree.right = r
                        all_trees.append(current_tree)
            return all_trees
        return generate_trees(1,n) if n else []