/**
 * 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) {
        while (root != nullptr && (root->val < L || root->val > R)) {
            if (root->val < L) {
                root = root->right;
                }
            if (root->val > R) {
                root = root->left;
                }
            }

        if (root == nullptr) {
            return nullptr;
        }

        TreeNode* prev = root;
        TreeNode* temp = root->left;

        while (temp != nullptr) {
            if (temp->val < L) {
              prev->left = temp->right;
                temp = temp->right;
            }
            else {
                prev = temp;
                temp = temp->left;
            }
        }

        prev = root;
        temp = root->right;


        while (temp != nullptr) {
            if (temp->val > R) {
                prev->right = temp->left;
                temp = temp->left;
                }
            else {
               prev = temp;
                temp=temp->right;
                }
        }
        return root;
    }
};