leetcode 95 不同的二叉搜索树 II
z
 2019-07-04 20:34:45  

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:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
# 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]
        """
        
        def helper(l):
            if not l:
                return [None]
            elif len(l) == 1:
                return [TreeNode(l[0])]
            ans = []
            for i, n in enumerate(l):
                for left in helper(l[:i]):
                    for right in helper(l[i+1:]):
                        root = TreeNode(n)
                        if left is not None:
                            root.left = left
                        if right is not None:
                            root.right = right
                        ans.append(root)
            return ans
        
        if n <= 0:
            return []
        
        return helper([i for i in range(1, n+1)])