Leetcode 95. 不同的二叉搜索树 II(Python3)

95. 不同的二叉搜索树 II

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。

示例:

输入: 3
输出:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
解释:
以上的输出对应以下 5 种不同结构的二叉搜索树：

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

---

# 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 generateTrees(self, n):
        """
        :type n: int
        :rtype: List[TreeNode]
        """
        if n == 0:return []
        res = self.dfs(1,n)
        return res
    
    def dfs(self,start,end):
        cr = []
        if start > end:
            return [None]
        if start == end:
            node = TreeNode(start)
            cr.append(node)
            return cr
        
        for i in range(start,end+1):
            ln = self.dfs(start,i-1)
            rn = self.dfs(i+1,end)
            for l in ln:
                for r in rn:
                    node = TreeNode(i)
                    node.left = l
                    node.right = r
                    cr.append(node)
        return cr