669. Trim a Binary Search Tree

https://leetcode.com/problems/trim-a-binary-search-tree/description/

/**
 * 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)  return trimBST(root->right, L, R);
        if (root->val > R)  return trimBST(root->left, L, R);
        
        root->left = root->val == L ? NULL : trimBST(root->left, L, R);
        root->right = root->val == R ? NULL : trimBST(root->right, L, R);
        return root;
    }
};