669. Trim a Binary Search Tree

[WhiteYin]

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

[WhiteYin]

递归方法，对每一个节点的值进行判断，

1. 先判空，
2. 然后若小于L则取其右子节点返回，
3. 若大于R则取其左子节点返回，
4. 否则对其左右子节点进行递归调用，
5. 返回该节点