669. Trim a Binary Search Tree（递归修剪二叉树）

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

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

class Solution:
    def trimBST(self, root, L, R):
        """
        :type root: TreeNode
        :type L: int
        :type R: int
        :rtype: TreeNode
        """
        if root is None:
            return None
        if root.val<L:
            return self.trimBST(root.right,L,R)
        if root.val>R:
            return self.trimBST(root.left,L,R)
        root.left = self.trimBST(root.left,L,R)
        root.right = self.trimBST(root.right,L,R)
        return root