Given a binary search tree and the lowest and highest boundaries as L and R, trim the tree so that all its elements lies in [L, R] (R >= L). You might need to change the root of the tree, so the result should return the new root of the trimmed binary search tree.

Example 1:
Input: 
    1
   / \
  0   2

  L = 1
 

  R = 2

Output: 
    1
      \
       2
Example 2:
Input: 
    3
   / \
  0   4
   \
    2
   /
  1

  L = 1
  R = 3

Output: 
      3
     / 
   2   
  /
 1


這道題描述的需求是：
給我們一個二叉排序樹 最小數l 和最大數r
我們需要做的是對這棵樹進行裁剪，讓樹裏所有的節點值都滿足在l和r之間

思想：
二叉排序樹的特點是：對於任意一個節點，左子樹任意節點數值都比跟節點小，右子樹任意節點數值都比根大
所以考慮，對於任意一個節點，
如果值比l還小，那就應該拋棄這個根，去右子樹尋找新的根
如果值比r還大，那就應該拋棄這個根，去左子樹尋找新的根
 


這樣的操作進行遞歸就可以了

我的python代碼：

 1 # Definition for a binary tree node.
 2 class TreeNode:
 3     def __init__(self, x):
 4         self.val = x
 5         self.left = None
 6         self.right = None
 7 
 8 class Solution:
 9     def trimBST(self, root, L, R):
10         """
11         :type root: TreeNode
 
12         :type L: int
13         :type R: int
14         :rtype: TreeNode
15         """
16         if root is None:
17             return None
18         if root.val >R:
19             return self.trimBST(root.left ,L,R)
20         if root.val<L:
21             return self.trimBST(root.right, L, R)
22         root.left = self.trimBST(root.left,L,R)
23         root.right = self.trimBST(root.right,L,R)
24         return root
25 
26 
27 
28 if __name__ == ‘__main__‘:
29     s = Solution()
30 
31     root = TreeNode(1)
32     root.left = TreeNode(0)
33     root.right = TreeNode(2)
34 
35     print(root.val,root.left.val,root.right.val)
36 
37     root = s.trimBST(root,1,2)
38 
39     print(root, root.left, root.right)

leetcode算法：Trim a Binar Search Tree