Tree-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

** 
 * 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 == nullptr) 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;  
    }  
};