vector<int> preorderTraversal(TreeNode *root)

      {

          vector<int> result;

          stack<TreeNode *>s;

          TreeNode *p;

          p = root;

          s.push(p);

          while (!s.empty())

          {

              p = s.top();

              result.push_back(p->val);

              if (p->right != nullptr)s.push(p->right);

              if (p->left != nullptr)s.push(p->left);

          }

          return result;

      }