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

题解：

class Solution {
public:
  static TreeNode* Trim (TreeNode* root, int L, int R) {
    if (root == NULL) {
      return NULL;
    }
    if (root->val < L) {
      return Trim(root->right, L, R);
    }
    if (root->val > R) {
      return Trim(root->left, L, R);
    }
    root->left = Trim(root->left, L, R);
    root->right = Trim(root->right, L, R);
    return root;
  }
  TreeNode* trimBST(TreeNode* root, int L, int R) {
    TreeNode *p = Trim(root, L, R);
    return p;
  }
};