669. 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
题目含义：给定一棵二叉搜索树和最小最大边界L和R，修剪这棵树，使得其中的元素位于L和R之间。

 1     public TreeNode trimBST(TreeNode root, int L, int R) {
 2         if (root == null) return null;
 3 
 4         if (root.val < L) return trimBST(root.right, L, R);
 5         if (root.val > R) return trimBST(root.left, L, R);
 6 
 7         root.left = trimBST(root.left, L, R);
 8         root.right = trimBST(root.right, L, R);
 9 
10         return root;        
11     }