bstFromPreorder

/**
 * 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* insert(TreeNode* root, int num) {
        if (root == NULL) {
            return new TreeNode(num);  // create new node and return it
        }
        
        if (num < root->val) {
            root->left = insert(root->left, num);  
        } else {
            root->right = insert(root->right, num); 
        }
        return root; 
    }

    TreeNode* bstFromPreorder(vector<int>& preorder) {
        TreeNode* root = NULL;
        for (int val : preorder) {
            root = insert(root, val);
        }
        return root;
    }
};