new_contents: // Time:  O(n)
  // Space: O(h)
  
  /**
   * 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:
      TreeNode* trimBST(TreeNode* root, int L, int R) {
          if (!root) {
              return nullptr;
          }
          if (root->val < L) {
              return trimBST(root->right, L, R);
          }
          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;
      }
  };
  
subject: Create trim-a-binary-search-tree.py