python--leetcode669. Trim a Binary Search Tree

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

题目意思就是给定一棵二叉排序树，以及一个最小值和一个最大值，让你把树中低于最小值和高于最大值的数删了，并返回此时树的根节点。

这一题用递归做，思路：当node.val> R时，此时修剪后的二叉树必然出现在节点的左边。 类似地，当node.val  

代码如下：

class Solution(object):
    def trimBST(self, root, L, R):
        def trim(node):
            if not node:
                return None
            elif node.val > R:
                return trim(node.left)
            elif node.val < L:
                return trim(node.right)
            else:
                node.left = trim(node.left)
                node.right = trim(node.right)
                return node

        return trim(root)