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

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     TreeNode* trimBST(TreeNode* root, int L, int R) {
13         if (!root)
14             return nullptr;
15         if (root->val < L) 
16         {
17             return trimBST(root->right, L, R);
18         }
19         else if (root->val > R) {
20             return trimBST(root->left, L, R);
21         }
22         else 
23         {
24             root->left = trimBST(root->left, L, R);
25             root->right = trimBST(root->right, L, R);
26             return root;
27         }
28     }
29 };