leetcode 95. Unique Binary Search Tree ii

题目要求

给定一个整数 n，生成所有由 1 … n 为节点所组成的二叉搜索树。

解题思路

所以我们节点是从1到n,当一个节点为val那么它的左边是<= val,右边是>=val,

我们用递归左右子树一步步解决!

 for l in helper(start, val-1):for r in helper(val+1, end):

主要代码python

# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = Noneclass Solution(object):def generateTrees(self, n):""":type n: int:rtype: List[TreeNode]"""def helper(start,end):res = []if start > end:return [None]for val in range(start, end+1):# 按BST要求划分左边都是小于val的，右边都是大于val的# 递归操作for l in helper(start, val-1):for r in helper(val+1, end):# 建树过程root = TreeNode(val)root.left = lroot.right = rres.append(root)return resreturn helper(1,n) if n else []

原题链接：https://leetcode.com/problems/unique-binary-search-trees/submissions/