class Solution:
    def generateTrees(self, n: int) -> List[TreeNode]:
        def generateTrees(start, end):
            if start > end:
                return [None,]
            
            allTrees = []
            for i in range(start, end + 1):  # 枚举可行根节点
                # 获得所有可行的左子树集合
                leftTrees = generateTrees(start, i - 1)
                
                # 获得所有可行的右子树集合
                rightTrees = generateTrees(i + 1, end)
                
                # 从左子树集合中选出一棵左子树，从右子树集合中选出一棵右子树，拼接到根节点上
                for l in leftTrees:
                    for r in rightTrees:
                        currTree = TreeNode(i)
                        currTree.left = l
                        currTree.right = r
                        allTrees.append(currTree)
            
            return allTrees
        
        return generateTrees(1, n) if n else []

树，中等

0094.二叉搜索树的升级版，递归实现构造二叉树

https://leetcode-cn.com/problems/unique-binary-search-trees-ii/