vector<vector<int>> levelorderTraversal(TreeNode *root)

      {

          vector<vector<int>> result;

          vector<int>tmp;

          //通过两个queue来区分不同的层次

          queue<TreeNode *>current,next;

          TreeNode *p = root;

          current.push(p);

          while (!current.empty())

          {

              while (!current.empty())

              {

                  p = current.front();

                  current.pop();

                  tmp.push_back(p->val);

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

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

              }

              result.push_back(tmp);

              tmp.clear();

              //交换并将next清空，

              swap(current, next);

          }

          return result;

      }