Leetcode 之Binary Tree Preorder Traversal（42）

 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;
      }