Leetcode 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 == NULL){
            return NULL;
        }
        if (root -> val < L){
            root -> left = NULL;
            return trimBST(root -> right, L, R);
        }
        else if (root -> val > R){
            root -> right = NULL;
            return trimBST(root -> left, L, R);
        }
        else{
            root -> left = trimBST(root -> left, L, R);
            root -> right = trimBST(root -> right, L, R);
        }
        return root;
    }
};