Given a binary search tree and the lowest and highest boundaries asLandR, 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

依然递归，分别对左右子树调用

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if(root == null)
            return null;

        if(root.val < L)
            return trimBST(root.right, L, R);
        if(root.val > R)
            return trimBST(root.left, L, R);

        TreeNode left = trimBST(root.left, L, R);
        TreeNode right = trimBST(root.right, L, R);

        root.left = left;
        root.right = right;

        return root;
    }
}