[Leetcode]@python 95. Unique Binary Search Trees II

题目链接

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

题目原文

Given n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example,
Given n = 3, your program should return all 5 unique BST's shown below.

题目大意

给出一个数字n，用1-n构建二叉查找树，返回所有符合条件的二叉树

解题思路

使用dsf进行求解（求个数一般使用递归，若枚举可考虑使用dfs）

代码

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

class Solution(object):
    def dfs(self, start, end):
        if start > end:
            return [None]
        ans = []
        for val in range(start, end + 1):
            leftTree = self.dfs(start, val - 1)
            rightTree = self.dfs(val + 1, end)
            for l in leftTree:
                for r in rightTree:
                    root = TreeNode(val)
                    root.left = l
                    root.right = r
                    ans.append(root)
        return ans

    def generateTrees(self, n):
        """
        :type n: int
        :rtype: List[TreeNode]
        """
        if n == 0:
            return []
        return self.dfs(1, n)