Solution2.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* trimBST(TreeNode* root, int low, int high) {
        while (root && (root->val < low || root->val > high)) {
            root = root->val < low ? root->right : root->left;
        }
        if (!root) {
            return root;
        }
        TreeNode* node = root;
        while (node->left) {
            if (node->left->val < low) {
                node->left = node->left->right;
            } else {
                node = node->left;
            }
        }
        node = root;
        while (node->right) {
            if (node->right->val > high) {
                node->right = node->right->left;
            } else {
                node = node->right;
            }
        }
        return root;
    }
};