669. Trim a Binary Search Tree

// Time Complexity: O(n), n is the number of nodes
// Space Complexity: O(n), to create n functions
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;
    }
};