1. 不同的二叉搜索树 II
给定一个整数 n ,请生成并返回所有由 n 个节点组成且节点值从 1 到 n 互不相同的不同 二叉搜索树 。
class Solution:def generateTrees(self, n: int) -> List[Optional[TreeNode]]:if n == 0:return []return self.generateTreesHelper(1, n)def generateTreesHelper(self, start, end):if start > end:return [None]all_trees = []for i in range(start, end + 1):left_trees = self.generateTreesHelper(start, i - 1)right_trees = self.generateTreesHelper(i + 1, end)for l in left_trees:for r in right_trees:current_tree = TreeNode(i)current_tree.left = lcurrent_tree.right = rall_trees.append(current_tree)return all_trees
2. 不同的二叉搜索树
给定一个整数 n ,求恰由 n 个节点组成且节点值从 1 到 n 互不相同的 二叉搜索树 有多少种?
class Solution:def numTrees(self, n: int) -> int:dp = [0] * (n + 1)dp[0] = dp[1] = 1for i in range(2, n + 1):for j in range(1, i + 1):dp[i] += dp[j - 1] * dp[i - j]return dp[n]