/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
 enum Flag { Left, Right };
 TreeNode* trimBST(TreeNode* root, int L, int R) {
  if (!root) return root;
  if (root->val < L) return trimBST(root->right, L, R);
  else if (root->val > R) return trimBST(root->left, L, R);
  root->left = trimBST(root->left, L, R);
  root->right = trimBST(root->right, L, R);
  return root;
 }
};