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.

    3
   / \
  0   4
   \
    2
   /
  1

  L = 1
  R = 3

Output: 
      3
     / 
   2   
  /
 1

解法：好像沒什么說的，遞歸

# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def trimBST(self, root, L, R):
        """
        :type root: TreeNode
        :type L: int
        :type R: int
        :rtype: TreeNode
        """
        if not root:
            return None
        if root.val < L:
            return self.trimBST(root.right,L,R)
        elif root.val >R:
            return self.trimBST(root.left,L,R)
        else:
            root.left = self.trimBST(root.left,L,R)
            root.right = self.trimBST(root.right,L,R)
            return root